ZSS is buffed because she has her armor pieces for a larger portion of the match.
Ice Climbers, in the current metagame, are buffed by this change as well. Considering the difficulty of getting a grab and performing the CG without mistakes, lowering the number of times you must pull it off per match makes it easier for the Ice Climbers to win this match. With 2 stocks, it would only take 2 grabs to win. If the set size is lengthened to compensate for the lower stocks, however, this won't change the Ice Climbers because they will need to win more games anyways. Thus, more grabs.
Lucario is buffed because his aura doesn't screw him over when he has a significant lead. If he's losing by enough for his full aura to take effect, he's going to lose anyways. If he does start to make the comeback, the aura buff lessens anyways, so this just makes it EASIER for him to come back.
PT I believe is also buffed by this. For matchups where you'd rather ignore Ivysaur, you can easily do so without having to switch at all. Start as Charizard, do as much as you can do, and then lay the hurt down with Squirtle after you die.
Ice, you're overreacting a little bit, don't you think? Lowering the stock count by one does not make you relearn PT from scratch at all. Everything still works how it did before the stock change, all you have to do is change your order. If that means starting as charizard instead of squirtle, that's fine. The in-game mechanics are still the same after that.
Also, hi PTs