[nickik]

I can answer some of that, 2024 is the current limit. But 2028 is being considered and I think its politically likely.

It could probably be extend even further but it would require incensing maintenance and expensive re-qualification.

NASA already has a plan however. They have a contract with Axiom Space to extend the ISS with new modules. After they have about 4 new modules they should eventually decouple after ISS and be a free floating station.

NASA already has a program in planning for a commercial space station. The same way they did Commercial Crew and the moon lander. They have already asked companies to come up with plans for a station. SierraNevada really wants that contract and SpaceX will probably bid something, probably others. These would be free floating privately run stations

So NASA basically hopes that sometime between 2024 and 2028 there will be two new stations and then its politically easier to drop ISS.

Crew Dragon can still do free flying missions (as they will do later this year) but SpaceX hopes to replace it with Starship.