Consider Game of Thrones. Are seasons 1-3 still worth watching even though 4-6 are mediocre and 7-8 are dumpster fires?
As someone who enjoyed the show for a long time and watched it all the way through, I'm not sure the answer to that is obvious. They wrecked it so badly in the final episodes that I don't currently see myself doing the marathon rewatching thing with it. I'm also not sure I'd recommend it to a friend who hadn't seen it, knowing it was setting them up for such a huge disappointment.
You got burned because you watched S7 and S8 and you have a grudge against D&D. But S1-S6 developed many good characters and plotlines, and it's OK leave a world in progress. Especially in GoT world which is all about treachery and turmoil -- the idea of a tidy ending for everyone is absurd, unless everyone dies. So someone new knows to watch only S1-S6 and it's a good show.
But S1-S6 developed many good characters and plotlines, and it's OK leave a world in progress.
Some people don't mind that. Personally, I enjoy stories with closure. I find it a huge downer to know the canon ending sucks and will ruin the plot for many characters I have been following.
If nothing else, skipping the final episodes means I have to decide when to stop watching. In the case of GoT, the end of season 6 is one possibility, but arguably the worst shark-jumping happened after The Long Night in the middle of season 8 and watching that far does resolve some of the other plot lines.
My position is that the problems exist well in advance of season 8 but that in seasons 5/6 it still felt as if there was enough runway for things to land safely.
This turned out not to be the case. Partially due to creator disinterest (by some accounts, HBO was essentially pushing to give them 10+ seasons with generous budgets) and partially due to a writing style that prioritized "big moments" over character/world building and consistency.
I think this is best exemplified by Arya's "House of Black and White" arc (light spoilers) in season 6. The arc setup is that the Faceless master tells her constantly "To become Faceless you must discard your identity completely -- become no one" while Arya secretly refuses to discard the literal symbol of her identity (her sword, Needle, hidden near the temple). This tension is then essentially just ignored and seemingly forgotten for the entire arc which ends with her being accepted by the master as a Faceless Woman (as "no one") while also maintaining her identity as Arya Stark (by keeping Needle). Nothing in her story ever refutes, upholds, or explores this contradiction. Like many other potentially interesting things, it is brought up superficially (possibly by accident) then lost in the shuffle as the show rushes to the next "big moment".
As someone who enjoyed the show for a long time and watched it all the way through, I'm not sure the answer to that is obvious. They wrecked it so badly in the final episodes that I don't currently see myself doing the marathon rewatching thing with it. I'm also not sure I'd recommend it to a friend who hadn't seen it, knowing it was setting them up for such a huge disappointment.