I actually sleep on my back. Not that I don't believe you, but why should I try something firmer when I find this mattress to be very comfortable and never feel

I actually sleep on my back. Not that I don't believe you, but why should I try something firmer when I find this mattress to be very comfortable and never feel back pain? I assume that this causes a risk of future back pain - would you have any information about that specifically? Thanks a ton!