Adding inventory space won't solve the problem, because eventually you will still get full. The problem is that people are too lazy to manage their inventory so buying extra retainers allows them to be lazy for longer.

You can get all quest and achievement items again from the calamity salvager or achievement menu.

You can get craftable items again from the market board easily. You can get most loot easily from the market board or by unsyncing the content you got it from.

So you can safely get rid of most items because they are easy to get again.