The controllers are black boxes though, but you can get 3rd party units for Tesla batteries _today_. JerryRigEverything on Youtube built his own EV Hummvee on the military chassis using some Model X batteries and all 3rd party electronics.
It wasn't plug and play, but perfectly doable in a semi-well stocked home garage.
Just imagine what can be done with actual resources and R&D.
The current limiting issue isn't skill or technology, but available new batteries. They can't make them fast enough even for new cars, never mind for spare parts.
But when battery production ramps high enough, there's absolutely nothing preventing people from upgrading existing batteries with more modern tech - bigger battery capacity in the same space.
There will also be a huge industry converting "dead" EV batteries to in-house battery backups. A car with 50% battery degradation is a bit iffy, but the battery will still have tens of kWh of usable capacity. Enough to run a house for a day or two in a pinch - or just store cheap energy and sell it back to the grid when the price goes up.
If we're talking hypothetically, why not just expect someone to develop a battery that lasts forever?
It just takes time to build enough factories to produce enough.
It doesn't exist yet beyond some small enterprises because there isn't enough volume: even 10+ year old EVs are still going strong with very few battery issues. We need to wait longer to get any business going.
There was a dude in Finland who replaced and upgraded Nissan Leaf batteries, he had to stop because it was a) impossible to get new batteries due to huge demand b) ran out of customers.