@Geneva19 My guest bed isn't soft, and I highly doubt all 5.6 million Airbnbs around the world have soft mattresses. 🙂 It just might seem that way because the ones you've stayed at have. My guests have made many comments on how comfortable the bed is. What it actually is, is a 4" thick slab of high quality firm foam, with a softer high quality 2" foam on top of that, and a nice mattress pad.
If a guest liked a really firm mattress, they could just remove the 2" topper, but no one has seemed to find that necessary.
In fact, before I had that room set up for paying guests, I just had the firm foam on the bed without a topper. Some friends came to visit and found the bed foam so firm, they referred to that bed as "The plank".