Simple: Keep your old mattress until you're sure that you want to keep your new one.

Or, buy a new mattress before returning the one you didn't like. This way you only need to store a superfluous mattress for a short period of time.

I feel like these are solveable problems.

I have a not-small apartment and there still isn’t room for an entire extra mattress.

Keep it under the new one

I skate through on laziness. I offloaded my last mattress a couple of years ago, mostly intending to thence get a new one, but never quite got round to it. If we all just embrace our inner slackers, we can put an end to the problems of mattress disposal!