As to me the main thing is that is really anti-decentralized and that's why will never be done by crypto community. But as for technical possibility it is possible in theory by series of iterations of hardforks with integration of data from one ledger to another one. I don't see any reason in such merging and radically against it but can see a variant how technically it can be done.
I would be radically against it as well. However, imagine like one token promising very good returns if you throw your coins in there, it wouldn't get ALL the coins, but it could potential ask for all the coins, asking doesn't guarantee returns but they could at least be big. How? Well how do we do DAI?
We send money in, and instead have something in return, same logic, we could have a project that accepts you paying with everything, delete all of that, and instead give you their tokens that worth the same, and then we would all try to keep that high. It wouldn't work, people would hate it like you and I would, but it could definitely be done technically speaking.