Can I insist on the original routing upon flight cancellation?

You can ask that they route you the same way, but you can not insist. Your contract with the airline is to be transported from A to C, period. The fact that you choose the flight through B is not part of the contractual obligations.

When your itinerary is changed is such a fashion your choices are pretty much get a full refund or be re-routed. Most airlines allow you a window of opportunity, such as 24 hours from notification to choose the option or ask about alternatives. After that period they are free to lock in the change. Of course you can always ask at a later point in time, just no guarantees.

If you truly want that stopover, you may have to ask the airline to re-issue your booking to include a stop at B. But this might mean extra charges as the airfare with a stopover might be higher (or maybe lower as I got lucky with once) plus ticket change fees.

I'm elevating one of my comments on another answer to a full answer, because I think it's relevant...

You are attempting to carry out a variation on "hidden city ticketing," and many of the same caveats will apply to you.

You are trying to gain the benefit of a connection with a long layover in B while entering into a contract with the airline to convey you from A to C. This is "hidden city ticketing", regardless of the fact that you will continue the journey to C (whereas most hidden city ticketing users won't - they will throw away the second leg of the journey) because you are trying to gain the benefit of travelling to B at no additional cost to yourself (while options at additional cost may exist).

As such, you are in the same boat as other hidden city ticketing users - there are utterly no guarantees about B. None at all. And you can't require the airline to fly you via B either.

_{NB: I know I will get some pushback on the use of the term "hidden city ticketing" here, but it's valid - it applies equally to cases where you disembark the aircraft when you do not need to, and also when you book a connection and don't fly the subsequent leg. In this case, it's a variation, because the subsequent leg would be used but the intention is still to gain the benefit of flying to B, so it's comparable.}

You can insist all you want but that does not mean they will. However not insisting is guaranteed not to give your results you want. So in order to have a chance of getting what you want, you must insist. To do so nicely and politely can get results. Sometimes the person answering you cannot help but it is possible that another can.

Some anecdotes from similar situations I encountered within the last 2 years:

• Flight booked A -> B -> C with overnight at B. Non-refundable hotel purchased. Flight A -> B gets cancelled, given A -> D -> C. No compensation or refund. Airline covers stay at D though but lost cost nice hotel at B. Only spoke to one agent.
• Flight A -> B -> C -> D (different ones than above). Flight A- > B delayed, too late for connection for B -> C flight. Agent offers different schedule 6 hours later, resulting in 18 hour wait for redeye. Talked to two agent, insisted until reached manager and got A -> B -> D flight, arriving early.
• Flight A -> B -> C booked. Airline offers A -> D -> C with overnight at D. A is in the tropics with 25-30C temperatures, D is in the north with around -20C temperatures. Insist with airline, talking to 2 agents and get back A -> B -> C all 2 days later. Airlines compensates for issue. Insurance pays for flight delay. Total cost of trip becomes negative!

The point is that insisting can but does not always work. The airline is apparently not under obligation to keep the routing as booked, only the origin and destination. The key is to insist on your issue while showing them need and flexibility from your side.

In my experience, following a major schedule change, most airlines will happily permit you to readjust your itinerary as you see fit, for free, regardless of the original fare rules.

For instance, that may include travelling earlier or later, the insertion or removal of a stop or layover, rerouting via different points, a change of origin or a change of destination, a change of flight on the same day, et c.

That freedom tends to be circumscribed as follows—

• You can usually only reroute on the same airline, or if airlines are in a close grouping, on those airlines. For instance swapping between British Airways and American Airlines going over the Atlantic is no problem, but swapping from Air France to rival carrier Lufthansa is not going to happen.
• If the city is no longer served by the airline, specific guidance for customers in your situation will be provided and may be quite restrictive. Usually that means interline carriage on a local carrier or a refund only.
• You can only change your itinerary once per schedule change. So try to get it right.
• The airline will offer you what they think is the best option. You need to be proactive and say "no, this doesn't suit me, I would like to book this alternative instead".
• The schedule change has to be significant (as a rule of thumb, over two hours, but it varies by airline and sometimes it is up to the agent's discretion). A cancellation forcing a reroute is always significant.
• You can't be totally unreasonable, it has to be broadly similar to your original ticket.
• Each airline has its own rules for exactly what changes you can make, it pays to be aware of this in advance.

My experience has been built up from exactly the situation you have. I buy a lot of fares that prohibit stopovers altogether, or would incur large additional taxes by including them (such as UK air passenger duty, currently an eye-watering £150 for a stopover). I also like to nest lots of tickets together, and if one ticket in the middle is changed it can ruin the timing of the other tickets. So I suffer at the hands of schedule changes and cancellations. They happen. But I have never had an agent say "our contract is A to B, we wash our hands of your stop at C"—rather, it is more along the lines of, "yes sir, your proposed rerouting is no problem, I will send it to re-ticketing, sorry again about the inconvenience".

It depends how you booked it. If you worked with the airline to specifically create an intended stopover at B for your use as a tourist... and that was part of your ticketing and contract... they need to get you to B.

(Stopover meaning an integral part of the travel plan agreed to with the airline.)

However if B was merely a super inconvenient connection -- then as far as they're concerned, they are doing you a favor by routing you differently and more efficiently.

There's another factor: immigration.

First you can have trouble getting B or D to land you, and the airline's obligatory preclearance only relieves them of civil liability, it is not final and you can still flunk your entry interview. In that case you will not be doing any tourism (unless they do one of those "not landing you, but not detaining you either, arrange your own hotel tonight" deals). Also, they may insist on deporting you back the way you came, rather than deporting you in the onward travel direction as you would hope.

But bigger than that, you may have travel restrictions, e.g. If D is Heathrow, you may not have secured a UK transit visa, or you may have no chance of securing one due to prior serial refusals. You may be able to use this as a bargaining chip: "Can't send me to D, there's a reason I routed through B". It doesn't need to be true.

Im not sure but if you clearly asked them , that you want to go A to C by passing B then they clearly cannot offer you new route without having B but if you asked flights to A to C then you cannot.