I don't think Blaziken or Greninja are a precedent for this, as other pokemon get both Speed Boost and Protean, whereas no other pokemon gets Power Construct. There is absolutely nothing that's negatively effected by banning Power Construct.
I really, really don't think anyone would care about that. The ban isn't complex. Just don't use power construct in OU. Only one pokemon can have power construct in the first place, which is broken on it, so yes, I would say it is a broken ability. Banning power construct but not Zygarde would make more people happy in the long run, because Aura Break Zygarde could still be used in OU.
EDIT: Besides, you can't even say power construct would be broken on another pokemon. It's one of those "change to another forme when a certain condition happens" ability, so it literally wouldn't work on any pokemon that wasn't Zygarde.
It's much less about who cares about it or who's affected by it, and much more about the philosophy behind bans. Abilities (and Moves) have been banned when the actual Ability itself is overpowered or uncompetitive, not in how it interacts with certain Pokemon. Moody and Shadow Tag (and Mimimize) fit these bills perfectly. Power Construct by itself is neither of these. Bans on Pokemon have occurred when they have access to a combination of moves, stats, and abilities that are deemed overpowered - we haven't banned just specific moves or specific abilities on specific Pokemon.
As well, a Power Construct Ban opens up the floodgates to a number of different Ability Bans. And there are precedents set already, even if we exclude Blaziken and Greninja.
Gen 4: Can we test Yanmega in UU if we ban Speed Boost?
Gen 5: Are Gothitelle and Wobbuffet allowed into UU if we banned Shadow Tag?
Gen 6: Is Medatite allowed into LC if we ban Pure Power?
There's a ton of hypotheticals we can throw around in the current gen too.
Taking a look at all the Tapu's, if any of them are ever banished to Ubers (or BL in Fini's case), can they be used if we just ban the appropriate Surge ability instead?
If Talonflame ends up in BL, do we allow him in UU if we ban Gale Wings?
If Bruxish ends up in BL2, do we allow him in RU if we ban Dazzling?
If Drampa ends up in BL3, do we allow him in NU if we ban Berserk?
If Zangoose ends up in BL4, do we allow him in PU if we ban Toxic Boost?
If Power Construct instead of Zyguard is banned from OU because it's too broken, then all of these and more become fair game.