Let's pretend MP and the core devs are vanquished by the Gavinista insurgency.
Now we all have XP, like or not.
Then something breaks at 3am. Who is going to fix it? The spurned former core devs? Gavin the Economist? Mike 'Let's reduce fees by 10x and induce crapfloods' Hearn?
Or do you imagine Gavin can simply post to r/bitcoin and crowdfund some new devs via Mechanical Turk and Fiverr?
Putting your coins on the longer chain won't save them, if that chain's blocks are invalid. We just learned that lesson with BIP66.
If the "insurgency" as you describe occurs, do you really think that the 'former core devs' will quit working for MIT, BitPay and Blockstream since they're no longer maintaining Bitcoin Core, an unused fiefdom? BitcoinXT might go nowhere - it might never get more than 10% of the network. However, as soon as it gets north of 40%, I predict that negotiations happen and all of a sudden the >1MB "gavin patch" goes into Bitcoin Core.
The core devs will continue to be core devs and XT will exist as a separate software fork, just the same as it is today.