I have relatives that have spent $60/mo for a storage unit for 12 years for some furniture they "might want to use some day if they buy a new house". Even pointing out that the furniture will likely look extremely dated by now and they've spent almost $10k so far doesn't seem to click at all.

I’ve just moved house. I have a few high quality pieces of furniture that I’d like to keep but don’t have space for, so considered storage. They are things that I’ve bought second hand and at auctions, and part of the pleasure is the hunt. So I felt I’d invested both money and time.

But then I realised that I can sell them for pretty much what I paid for them, put that money into a savings account or investments and buy them same things back in a few years. So that’s what I’m doing.

And, of course, I probably won’t buy the same things back in a few years. If I do then I’ll have all the pleasure of the hunt again. Or maybe I buy something else. Or maybe I just have the cash sitting and I can count my gold, Smaug style.

But either way I won’t have spent thousands on storage.

That realisation was extremely liberating.

Basically, don't rent a storage locker unless you have a fairly specific exit plan. There may be exceptions like I have no intention of moving out of a condo/apartment but I do sports that require fairly large equipment. But it's a good rule-of-thumb.

But now it's a sunk cost, might as well keep it going, right?

Yeah, that's what you think the storage locker is for.