This is why Smart TVs are terrible products.
"Cloud Rot" is a clever term for this. Products that rely on software support or online service lose functionality when the manufacturer stops supporting it.
Google shouldn't let OEM's control updates period.
No because not a single TV maker will want to bother with that. 4k budget android TVs are starting from like 250euros nowadays, if you expect 5 year of support for that price you are delusional. Premium TV makers should support for longer, but that's up to them.
Only if they do it for their devices first.
5 years would be too long, this would hold back the ability to build more powerful features into the OS. 3 years would be more realistic.
Smart TVs are not something we should support. TVs should be something that we can plug things into for many many years.