Body checks didn't 'worm their way into Extremes', they've been there in some form since Thornmarch and Striking Tree Extreme at least.
They just deliberately tried to make them less punishing since Sephirot (the actual term the developers themselves used is "long-rope jumping" -- if someone else trips, it trips everyone).
I think the result is that they've kind of run out of ways to make the extremes feel different and that's why we're seeing a return of old fight design like more faced paced movement in Barb, Body checks in Golbez and phases based on %HP in the recent one.
Extremes are in no way now Savages. The hardest extreme trials have ever been was 3.1 thru 3.3, Thordan, Sephirot, Nidghogg, with honorable mention to Shinryu in 4.1. They've actually been made significantly easier overall starting with 3.4 and that was their stated goal. If anything, they're more casual today than their inception, as they were originally envisioned to be a difficulty beyond the original 3 'hard mode' trials, which is closer to what extremes are today if it weren't for the ridiculous amount of ilvl and action potency we have on top. Fights that will kill you and require some strategy and attempts to succeed, but easy to farm if you know what you're doing.
Original extremes were somewhere between t2 and t4 in difficulty, that is, a floor 1 and 2 of a savage when compared to today. In fact, their reward structure was also originally weekly instead of 'beat me 5 times for a weapon' or 50 times for a guaranteed mount.
I think this reward structure reflects the fact that their intended purpose has shifted to be more casual, only really there because of tradition and so people can get a shiny mount rather than being a fight for the battle designer's creativity's sake.
I think the worst outcome of this is that they've stopped being a bridge for the difficulty gap between normal and savage. Barring relatively easy first floor fights like o1s and e1s, the gap is a bit large now and I imagine it's daunting for someone interested in taking the plunge.
But there's also only so many ways they can make fights harder now, because you can't lose aggro, you can't misposition a boss, you can't attack the wrong target, you can't run out of TP and barely can run out of MP, you have tanks and dps that can heal the party, you can't miss a boss tell, you don't have to dispel anything, you can ignore every knockback, you never have your hp or defense reduced from death and you're never out of healing range.
Maybe all they can do is super complicated arbitrary bullshit like limit cuts and overlapping Aoes that all resolve at once leaving only the one safe spot, and that IS too hard compared with what made fights challenging before. If that's the case, then my sympathies, that's the game that we have now because all the RPG elements have been relegated to social Role Play, and more and more action and recognition elements are crammed and stacked into the battle content to compensate.