I don’t think people are objecting to the central idea - giving more options to more mounts sounds great (to me, at least).
But taking something earned away doesn’t feel good, and sets a very bad precedent.
Not that I think it would happen - but imagine that you really dig Zandalari trolls. They were why you bought BFA, and you hit the rep grind as hard as you could to unlock them ASAP. And once you unlocked them, you race-changed all 12 of your max-level characters to Zandalari.
But then let’s say that tomorrow Blizzard decided that Zandalari trolls are “overpowered” based on the logic that “you see so many of them.” So they say that in the next expansion, they’re removing the ability to play any Zandalari trolls except Paladins. Otherwise there are just “trolls” again now. Sucks if you wanted to use the Zandalari on any other type of character - it’s what you spent your money to do, grinded rep for, etc.
Naturally, this causes a huge outcry, and they back away from it. Everyone feels relieved. Bullet dodged.
But then, during the NEXT expansion, they announce that now EVERYONE can play all allied races WITHOUT grinding rep. That sounds pretty cool, right? Options! No grinding! They’re doing us a favour! And even if you’ve done the rep grinds already, you don’t begrudge anyone the ability to not have to do them.
But then they announce that they’re adding the limitations to the Zandalari that they tried to foist off last time - they can only be Paladins now.
But it’s cool, because you CAN buy player-crafted tokens to unlock the ability to play other Zandalari classes. And hey, you’ll get one for free every time one of your alts grinds out Honorbound rep. Or, if you’ve already ground out the rep previously, you’ll be able to buy tokens to unlock the other classes really, REALLY cheap.
The question is: does all of that sound cumbersome to you compared to how things are done right now? Do you think they’d do that? Why would they NOT do that?
If the answer is “because it would make players feel screwed over,” you have the answer as to why people are worried about this whole water strider thing. It COULD be really simple and fun (equipment to allow more mount freedom of choice), but it’s burdened with strange extra conditions that make it feel in no way “clean.” It feels like trick to get what they wanted to begin with, just in an underhanded way.
And before you say “they wouldn’t do that!” - think about how players discovered that mobs were suddenly scaling health back in Legion, after they’d previously announced mob scaling wouldn’t be something that would be put into the game. That wasn’t announced - it was snuck in.
There’s precedent to be concerned. It feels like attempt #2 of “let’s get rid of water-walking in the next expansion” more than it does “let’s give everyone water-walking from now on.”
Your mileage/tinfoil-wearing on such things may vary, of course.