The biggest issue with the golden mounts as gil sink is just that they're not going to do their job. People who can already afford them will just buy them day one and then go back to complaining that there's nothing else to buy. Or, as evidenced in this thread, they'll complain that the thing to buy isn't the thing they want to buy. Also, for those who already have the money to blow on a golden mount "gil sink" chances are they have the ways and means locked in to just make more money so even if the mounts do take a chunk out of their bank account they'll just make it back and, again, complain they have too much gil.

It's also worth considering just what "expensive" even means. YoshiP can go around saying they are really expensive but keep in mind he also says that the "new dungeons are challenging" every time new dungeons come out. Either way, we have players in game who literally have a billion gil banked. There is just no way that the "expensive" gil sink mounts are going to actually be priced in a way that will impact players like that. "Expensive" is relative.

As for some of the other suggestions in this thread, I think players need to keep in mind that more functional gil sinks can be a problem in that they can end up punishing the players who aren't rich. And if the gil sink is too appealing/useful/functional it can actually end up encouraging RMT and gil selling.