I had something similar happen last year, but with Delta.
unfortunately travel insurance won't cover this as a covered reason (only airline default/bankruptcy) for Trip Cancellation.
IF you had started the trip (and they ceased service on your return flight), then *possibly* the Trip Interruption/Delay would kick in, but even then they will typically only cover the refund of the tickets, they won't pay the price difference (and let's face it, booking a new return like 3-6 days before is going to be super expensive).
That being said:
In my experience, even booking another airline may not yield the results you want. In my case, not only did Delta cease flights to/from CZM (except on Saturday), but within a matter of 3 months, AA and UA changed all their flight schedules so that *all* the flights out of CZM (it was our return that got whacked) were 2-day trips home (spend like 10 hours in IAH or DFW).
So I'd say be prepared for a "worst" case in that regard.
We ended up having to spend about an extra $100 and 3+ hours to get re-routed back through CUN.