You keep saying that this is a major change. 1-2 slots is not a major change.
1-2 slots can absolutely be a major change. And this isn't 1-2 slots, it's 1-2 slots, 16-20 KB protection for every character, and 20% slow resistance with Winter's Gift, which is huge considering how easy it is to build for slow resistance now with the winter event sets.
As others have repeatedly stated in this thread, there are likely very few people actually slotting that much KB protection in PvE builds. In reality what will probably happen is most people will ignore those extra slots because they probably won't even go to the P2W vendor and get the extra prestige sprints.
All my PvE builds have one KB IO slotted in Combat Jumping and I can count on one hand the number of times they have been KB'd. I don't take CJ just for the KB protection though, I take it because CJ+Hurdle is the best in-combat movement you can get in this game short of Inertial Reduction or Gymnastics.
I'll also address your earlier suggestion of adding KB protection into the inherent base resists that everyone gets. There are two issues with that:
1. Not every AT gets inherent base resistance in PvP.
2. To turn your argument back the other way... if I suddenly have all that extra KB protection for no investment I've now opened up at least eight slots to do other things with (and this isn't theoretical, you actually need eight slots for KB IOs in addition to Acrobatics to reach the level of KB protection you need to be survivable). By your own earlier argument that's a massive balance problem.
This is all kind of dumb anyways, because extra KB protection wasn't even a thing before I12 when Acro's KB protection was lowered from 100 points to 9 points. You can go on all you want about how you're basically gaining free slots, but the way it used to work meant some builds (specifically PvE ones, because every PvP build is going to be taking that pool for CJ and SJ anyways) would have to compromise into taking three power picks just to get any KB protection at all. The very existence of KB protection from IOs removed that requirement and gave builds more wiggle room. I'm having a very hard time understanding why anyone reasonable would be opposed to this.