This is an interesting question. It depends on the firmness and material quality of the mattress in question. All things equal, the box spring does provide some “give” which means less strain on the mattress over time. To keep its shape on a platform, the mattress would likely need to be flipped more often — maybe 6-7 times a year instead of the usual 4. I’ve never heard friends who have them complain about losing shape more quickly though.

Personal note: I was considering a platform bed about a year ago because it would’ve looked cool in my master bedroom. I’m 5′ 11 and 165 lbs but it was tough for me to get up and out of. I wasn’t sure that was how I wanted to greet the morning. Lol. I went with a shorter box spring and mattress instead.