I'm a little all over the place with this question, I don't quite know how I want to answer it.
Things like Game of Thrones and Halo have gone ****, but despite how much Halo 4 dropped the ball and how criminally bad Halo 5 was, I wouldn't trade my time with the series before that for anything. Same with Game of Thrones. These last few seasons have been bad (since they ran out of material from the books and started writing original storylines, coincidence?) but the books are really good, the world is really rich, and the first four seasons are some of the best television I will likely ever see. I'll never not be glad I got into those franchises, even though they've faded to a shadow of their former selves. Although Halo might make a comeback with the MCC coming to Steam kednfujihfiowreikwrfeaio
I don't know how to approach the question with regards to things I'm not into. Like, I suppose I'm glad I never got into something like Final Fantasy because there are so many and it would have eaten up thousands of hours of my life, but then if I were into Final Fantasy then I'd have wanted it to do that, right? Anything I got into that ended up going **** would be disappointing but I'd always have whatever got me into it in the first place. The Elder Scrolls comes to mind. Skyrim is a heap of **** the breaks or ignores most of its own rules and lore, none of the writing in it makes any sense, and the gameplay is so stripped bare that the only interaction you have with anything is hitting things until they stop moving. I have absolutely no interest in The Elder Scrolls 6 because, given Bethesda's track record, it can only be even ****ing worse. Despite all that, Morrowind and Oblivion, which also have their share of issues, are games I'll always treasure. Not just because of the adventures I went on in them, but because of the friends I bonded with through them and the memories and stories we still tell today of things we did in Vvardenfel a decade ago. Skyrim can stop me making more memories like that, but it can't take away the ones I already have. So, going back to Final Fantasy, even if, say, 10, 12, and 13 were all crap and made me resent Square, I'd still have those memories and experiences of, say, 6, 7, and 8 that wouldn't be affected, and if I liked those enough to play a half dozen others then, presumably, I'd have a similar response to them as I do to Morrowind or Halo 3 or The Battle of the Blackwater.
My thoughts would maybe be different for a franchise that's been going a long time, or even concluded, that I've never been into, but I don't know. Usually when I get told "X is good until season Y" or something of the like, it tempers my response and I still really get into whatever it is and recognise the drop in quality without feeling it in a harsh way. Thief would be an example. I first played the franchise a while after Thi4f came out, and all I heard was "This was once one of gaming's greatest franchises and now it's UNPLAYABLE ****!!!!!!!!". So I played it anyway, knowing the series would come to this diabolical end point that has likely killed it forever. The first two games almost instantly became two of my favourites of all time, the third is excellent, if a little fiddly at times, and Thi4f itself is... bad but I was able to go in knowing it would be, so the blow was softened immensely. I was able to focus more on the few things I liked than how it betrays the very design philosophy of the originals. I'm not glad I never got into Thief, I'm instead glad that I got into it despite knowing that it will culminate in a game many players see as a personal insult.
I guess my feeling on the matter, then, is that even if something goes bad, that doesn't make my initial love a waste, and anything good enough to become a long-term franchise most likely has something to like, at least initially, that will still be present. This is probably more the case with video games since each one is its own complete, separate package, so it's easier to ignore the bad parts. 343's Halo games haven't made Bungie's bad and haven't made the friends and memories I made playing them bad. Skyrim, if anything, has made me appreciate Morrowind and Oblivion even more. At the very least it hasn't taken away any of the things I loved them for. It can't. Even with Game of Thrones on track to end in a way that will make literally no one on Earth happy, all the time and emotion I invested in the earlier seasons is still worth it to me, partly because it was so good and partly because the worse season 8 is the more videos and essays people will make using it to explain and demonstrate story-telling technique, structure, character and theme development, etc. I don't feel like I wasted anything on Game of Thrones just because the last season is a let down, basically. My investment will be paid off, just in a different way. I'll learn more from it.
I don't know if I've made any sense here. Maybe it all boils down to the journey being more important than the destination or some crap.