Pillowy soft pillowtop, but will it rebound?

I've preferred Beautyrest mattresses for years, and have shifted to the pillowtop medium firm as my husband and I have gotten older. I have arthritis in my left shoulder, so sleep on my right side. My husband has sleep apnea, loves sleeping on his back, but has trained himself to sleep on his side. I love the comfort of this mattress, but the pillowtop takes some getting used to. Mainly, I worry that the soft sinking indentation that's there after laying in the same position most of the night will not rebound with repeated use (we've had it for 2.5 months), and we'll be left with a hilly mound in the center. Not sure what I'll do if that happens? In any event, so far we're enjoying it and sleeping well. No pinched nerves, no back pain, and we're not sleeping 'hot.'