Once you make changes to a chain its not easy to fix without making a huge mess. Hopefully the migration with be done by Q1 2022.
The migration is a decent amount of work but it's not the difficult research task that the beacon chain was. Someone did a proof-of-concept in about a day, though the exact plan has changed since then.
Likely closer to: they'll create a mirror on the new chain and a group of unknown size will keep ETH 1 fork alive.
Kind of like how Ethereum Classic (ETC) is subject to 51% attacks every other week and requires multiple days' worth of confirmations after deposit to any exchange.
They've been "moving to proof of stake" for something like three years.
The miners are making money off of them not moving to proof of stake. The miners stop making money when they move to proof of stake. The only people who can actually allow the chain to move to proof of stake are the miners.
Meanwhile, xtz and ada have been proof of stake since their inception, 3+ years already for both.
They've been _planning_ to move to PoS for ~3 years, with a much too optimistic timeline. In the meantime a lot of companies working towards that have collapsed or refocused on other efforts, delaying the whole transition. It will probably take another ~3 years (my personal estimate) until the transition is reasonably finished and core parts of the ecosystems have migrated, but at least it's moving into the right direction.
PoS has always been on the roadmap even before their ICO. But yes, with a MUCH more optimistic timeline.
2-3 years ago is when the Casper FFG paper was published and subsequently the architecture of Ethereum 2.0 (and the concrete 3-phase roadmap towards it) were published.
The miners can keep running the legacy version, and try to convince users and exchanges to stick with it. Given the massive community support for PoS, they're not likely to be very successful.
They claim they are going to fork ETH to make a POS fork of it. Now the people claiming this are going to want you to believe that’s the “real” eth- but the miners may not agree. They have no incentive to do so.
And yes, they can absolutely keep running the real chain and let the chips fall where they may. Given that course of action benefits them, and the other benefits Vitalik and his whale friends...
It doesn't matter what the miners believe, if the exchanges, applications, and users have moved to the PoS chain. Which is very likely to happen, given the community support for PoS, the scaling coming on that side at around the same time, and the fact that exchanges are already letting users earn returns by staking ETH, while the exchanges take a percentage of profits.
Vitalik et all have been talking about pos for years. Meanwhile a huge portion of the cryptocurrency space went there already (years ago) and left eth behind. POS isn't some new technology.
Fees don't come from whether it's pow or pos, they come from volume and the need to prioritize transactions.