TSM have certainly made a slew of changes in the last few years, but that feels more like were trying to build the right set of pieces rather than just rashly moving people around post-Zven + Mithy era.
TL changed very little apart from continuing to upgrade and won four splits until Broxah entered and Doublelift left.
And C9 have been constantly praised as continuing to cultivate and build talent and make changes pretty slowly while continuing to put up consistent results.
So idk if I agree that Top 3 NA is too liberal with roster moves, but you do make a good point I think: that there has been a LOT of change in the past year and there's already a lot more with Reapared and Licorice leaving C9 already. Although they kept their core together for so long I'm not sure it's fair to say that C9 haven't tried enough with the pieces they had.
TL;DR Not as much change in the Top 3 as you think, but recently (aka this year) a surprising amount.