worse case scenario you end up with same non smart TV.

Nope. You end up with a Smart TV still, but some of the stuff is broken, the interface is slow, etc. If Smart TVs had a way to actually turn it into a normal TV when it is past its prime, that would involve manufacturers showing some forethought that as of yet they have not.

You can still use them as a TV. Like how you can still use your multifunction printer as a scanner when it can't print anymore. Or how you can just buy a countertop microwave and try to ignore the one installed above your stove that no longer works. But it will never be as good as just having a single standalone product that is still 100% functional. It will always be that half-broken toy.

They are terrible because that is a predictable outcome. Expected.

They are terrible because they take a device that can easily last for decades without any problems and anchor it down with something that is EXPECTED to fail within a few years.

They are terrible because a non-smart TV with a relatively inexpensive and easily upgraded or replaced streaming media device is a far better choice.