As a thought experiment, I suppose you could:

1. Create a sidechain which is a Bitcoin clone, but with an entirely different consensus system.
2. Peg all BTC into the Sentinel chain.
3. On the sidechain, create a soft fork that makes peg-outs impossible (peg-out transactions on the sidechain go from valid -> invalid).
4. All economic activity moves to Sentinel chain and old Bitcoin fades away.

You've really made me think today, John. Thank you 😆