I watched Lost well after it came out (during the pandemic) and I thought the ending was fine. It wasn't a great ending, but it didn't ruin the show. It doesn't make the rest of the show terrible, just makes the ending weird but ok.

Similarly the ending of How I Met Your Mother, if you just ignore the last episode then the show is still great. Sure the ending is bad, but the show isn't ruined.

Now that's not true for Game of Thrones. That show builds to the ending and when that ending goes nowhere, or character motivations are completely wrong, that is a failure.

Now Dexter is weird. While the last season was kind of a mess, had Dexter just gone off into the sea and died, I guess that would have made sense. But really the whole season was rough, so anything they did wouldn't have worked. BUT! They left the door open. Recently they followed up. The newest Dexter has been pretty entertaining.