Tery_Are you implying that the lack of long-term teams is a newer issue? Because if so I got bad news for you...
It's not a new issue but I feel like it's worse than it used to be.
Partly I think that despite their issues, ESEA's roster moveup rules encouraged a lot more teams to move up as a team. If you were even one of the better players on a low playoffs open team, would you take your free spot on an IM team with your moveup roster, or take your chances LFTing for IM without a great e-resume?
Now there's almost no incentive to move up with your team unless you absolutely believe in them. Everyone wants to go at least 8-8 in their first season in every div now so they all go LFT I firmly believe this leads to moveup teams losing their best players more consistently, thus struggling and losing motivation to stay together.
Also (this is part of a whole other theory) I believe the rise of discord has killed the classic idea of strong friend groups/circlejerks, which were a major source of long term teams. When you hang out every night in voice with the same 8 dudes, you're generally gonna be making teams based around playing with those dudes for as long as its viable. Now people just post the same sped up gifs in the 23 identical tf2 discords they're in with the same 93 people in them. Season ends, team discords die, rinse and repeat.