There's technical limits I've been told we cannot exceed with the current inventory implementations, and we're getting close to that, but I've discussed it with the team and we're looking to bump up the max slots in the near future.
We definitely don't want evo materias of any kind to approach the value of 3 star XP materials, but if you have more than you could use and more gold than you can spend, then obviously evo runes and evo shards are effectively free XP, as insanely lossy as they are.
That said, we definitely are aware that a lot of players can easily save up millions of gold and not have anything meaningful to spend it on. We have some long term solutions, but we've been discussing solutions to implement in the interim as well (sort of part and parcel with the Iron Coins warchest problem, just with slightly different groups of people/possible rewards).