well you just added another point here, the forge, not hard to create 100s of items instantly and neededing said space to do it combined with everything else i said.
also ur arguement may as well be "once you have essentially completed the game and gear all ur champs you wont need the extra space" :P im getting a very mixed emotion of yes and no on this topic overall though. but still think the option to purchase additonal space if you want it could be a really good add on.
what the forge example really does is further show why everyone could stand to be a little more diligent in the type of gear they're keeping around. the forge is where i make most of my gains these days, and i think that its usefulness holds true for players who farm FW 20/21.
i would argue you should be plenty choosy - if not more so - early as well because of silver and pieces that are often going to represent diminishing returns in a hurry.
I am absolutely down for more filters that make it easier for folks to figure out what they'd like to sell. but I also think almost every single player would benefit from knowing what their stat thresholds are in sets/types of champs, what you actually need, what is junk and what isn't.
even if we're talking sets with strong pve applications (regen, reflex, provoke, etc.) or pvp (immu, stone skin, etc.) you are still going to want pieces that get you to the highest possible builds on your account, rather than just keeping pieces that are so very unlikely to make it into a meaningful build.