The problem begins when they'll start putting EVERYTHING on the cash shop: we have mounts and costumes already, which could be easily implemented in-game via events or whatnot. These costumes are nothing major, you're right, but then what happens when they'll start adding things that could potentially make the game pay to win? It will soon become mandatory and not just optional, making this game on par with SWTOR in terms of cash shop exclusive items and I'm sure as "eff" that nobody wants to copy that (even though I do like that game, just not as a f2p)
We have mounts and costumes for now...but then when they'll add exp boosts, special buffs or even a "possible" house on the cash shop, you have nobody to blame but the community: they were fine buying and buying the cash shop items so much, that SE realized this and might make everything be a paywall. It will be a pay-to-win game at this rate: maybe not tomorrow or next year, but at this rate of things and content delivered in the shop they might evemtiaòòu put more vital items such as weapons, armors and even additional customization options.
I mean, here's what they could think at one point:
"Hey, people are buying the mount because it's of a different color and has a barely noticeable music in it? Let's put more of that stuff!!" - "Wait, we could try putting this tiny little armor in the game and see if it sells!" - "WOW IT'S SELLING LIKE HOTCAKES, let's put weapons in it too!" - "Screw it, let's put entire armor sets: people buy anything we throw at them!"
And that's why we are complaining: it's not about the current mounts and items into the cash shop,
it's what they could lead in the long run.