A lot of things are good for the first few seasons because they don't have to explain things. Yet. See Lost for one of the more famous examples where the writers later admitted they had no idea wtf they were doing during the middle and were just coasting along on both hype and mystery boxes.

The problem with doing this is that for very popular projects that take too long to finish the hype from fans can turn toxic in ways other than simply fans hating on the material and artists/writers involved. When the hype for what comes next is so high it becomes impossible to ever meet then the very success that brought increased attention leads to its own downfall.

Look at Duke Nukem Forever that released in 2011 after 15 years of waiting from Duke Nukem 3D. It took too long that by the time the game finally came out it was never able to match the expectations of the fans who had mythologised what the release would turn out to be. Half Life 3 will suffer the same fate if that ever actually comes out. Sherlock experienced this after season 2. Arguably Brexit suffered from this too because it was never actually nailed down precisely what that was going to be and you ended up with a lot of smaller groups under the same banner who expected different things to take place, many of which would be incompatible with fellow supporters of the same cause.

As for AoT what made it popular was the characters fighting the Titans but when the story started trying to explain "there are bad guys on both sides" and moved away from the starting location it became extremely convoluted and detached from the original premise.

The pre-destination paradox problem didn't help things, either.