I grew up with LBT up to LBT VI. After that I had no idea they were still making more of these movies. I think it really hit me when I saw a trailer for LBT X I was like, wait, they are still around?
I personally don't mind the sequels up to LBT IX. The voice of Littlefoot during LBT V - IX was obviously getting older and it didn't sound that good in IX. Also, by that point, besides X, the plots were unoriginal.
I guess, like most people here have said, everybody remembers the original for being a masterpiece, but then all of the sequels are of nowhere near the same quality and therefore ruin the original.
I don't necessarily think they ruin the original, but I will not deny that they are nowhere near as good.