That’s a good point. With regards to the Finality Gadget, I doubt we’ll see it live on mainnet in <1 year (given that light clients for Eth2 need to be built and that work hasn’t started as far as I’m aware). This would come fairly close to the ~1.5 years in which the bomb would go off again.
Aside from the bomb, though, miners will have an incentive to upgrade to what is considered the “proper” Ethereum by the community.
If a large majority of Ethereum users & infrastructure providers consider the finality-gadget chain to be the correct one, then miners have an incentive to follow that chain given that it will be “ETH” and have most/all activity/economic value tied to it.