We have always laid rebar on PEX, it was suggested to me today that this would wear on the PEX. I would think no more than the aggregate in the concrete. Opinions?

My gut feel is the same as yours- there's not enough motion from the thermal cycling to generate actual "wear".

Both steel and any sub-fraction of the concrete mix is considerably harder and less flexible than PEX. Any micro-gaps between the PEX and steel/concrete will become imprinted on the surface of the more flexible soft compliant PEX, but it shouldn't go any further than that.

Rebar can have some sharp pieces that could potentially damage PEX. We tie the PEX to the rebar in such a way that it doesn’t come in actual contact with the rebar. Once the slab is poured and everything is locked in place, I would think all should be good even if the PEX does contact the rebar. We always pressurize the PEX with air prior to the pour and leave it that way until the system gets purged and filled with water to make sure the PEX doesn’t get damaged by any of the other trades during the construction phase. I suspect that you are more likely to experience damage to the PEX if you don’t properly sleeve where the PEX goes through control joints and these open up enough to cause problems down the road. PEX should have a 200-300 year life expectancy in a concrete slab according to the PEX companies.