For me, what I consider to be the classic runs of the titles I loved were those that took place when I first started reading. This is certainly true of LSH, Uncanny X-Men, New Mutants.
Yes, change is a part of life and I wouldn't want something to be set in aspic but change has to be managed the right way, be organic and it has to take fans along willingly, not kicking and screaming but most importantly, treat them with respect (Disney I'm talking to you re Star Wars cannon).
UXM was quite a different line-up after the Mutant Massacre and the team I had come to love over the past 40 odd issues was no longer but I stayed with it because I was still enjoying it and (for the most part) liked the new (to me) characters that were brought onto the team. Continuity was maintained and history wasn't being trashed. Where I lost interest was when the team went through the Siege Perilous and Psylocke became a kick-ass Ninja babe etc. I stuck with it for a while but I just wasn't enjoying it anymore and that, basically, is what reading a comic is all about.
To get back on topic with Legion, I started reading in the early 80's, just at the start of the classic Levitz/Giffen run and I stayed right through until the destruction of Earth in v.4 LSH #38.
Look how different 5YL Legion was to what had gone before but I still loved it because it was the characters I knew, just a bit older (which is what should happen in comics anyway, imho).
I stopped reading the title a) because I was losing interest in comics in general anyway and b) because I felt there was too much emphasis on the SW6 Batch and things were moving away from "my" Legion. Within a couple of years there was the first of what was has been many reboots, which has put me off trying to pick up the title again.
I am more than happy to read and re-read issues I know that I enjoy but sadly have no interest in any of these titles today (but I don't begrudge those who do their enjoyment).