An easy way to fix it would be to just oh I dunno let us go back to the DLC after we leave it... <_<
They'd have to make some of the things respawn. AFAIK there's only so many cosmic knives in the game world (granted there's a
lot of them). I think ghost people respawn, so there's a theoretically limitless number of spears and stuff though.
They could just make it so you can break down spears into their component parts too.
The bigger problem is what happens when you run out of police pistols or autorifles. For that, it might have been better to include a vending code for those, but which is only available after the end (or really expensive) to prevent people from spamming them.