Otherwise it would cheapen the appearance of storage chests and Eubicha.
Man, that's just a small quality of life. A minor detail for player comfort.
And expanding the vault by investing money that eventually has nowhere to go, which is not the case with unique loot and stuff, shouldn't be a problem on the logical side, it's just buying a couple bigger boxes.
As for Eubicha, it largely boils down to Skyrim companions being taken as mules to carry Loot. It turns out players will take her along for the journey not because they like her, but because it's a fucking walking storage space! Which is unfair to the lovely mare. And Jstar also mentioned that in the upcoming dungeon there will be a pool of tainted liquid, and at the bottom is a chest expanding the storage space. Except the only way to get it is to bathe in that sludge and get fucking shitload of corruption or make Drifa do it, which will almost certainly fuck her up. But it's a box! The rarest artifact in the Marches! So of course the players will do anything to get it.
I'm still convinced that buying something doesn't devalue getting the similar thing through quests or event, since it's free, and that's always nice. And the addition of some features aimed simply at convenience for players and not changing the balance of the game should not be questioned at all. I mean party shuffling wasn't THAT big of a hassle, but a lot of people appreciated being able to do it quickly and in almost any location. And many will appreciate the same opportunity with the vault.