This screenshot is from right after a Brotherhood wave in CMM. My inventory was empty (aside from that bottom row) before the wave.
Behind me is the rest of the loot that can't fit into my inventory.
This means that I can either leave most of it on the ground, go back and forth to the vendor and hope that the loot doesn't disappear, or sit there and open all the boxes, pick up more boxes and open them, and then pick up MORE boxes and open them until I have everything. At which point the next boss wave is starting and I have to do the whole thing all over again.
No one is asking for a loot nerf. But no one likes playing Marvel Heroes Inventory Management constantly.
Making cosmic worldstone caches, mm boxes, icp boxes, and reliquaries stackable by level would GREATLY reduce this problem. Mayhem boxes are already stackable; these other boxes should be stackable too.