The riverbed absolutely
is not supporting millions of gallons above it. The riverbed only supports the water
directly above it - For ~3 meters of average depth, that's something like 5 PSI.
The riverboat passing overhead actually reduces the load on any patch of the concrete bed since it's displacing water with a less dense material.
@lazyboy97o is absolutely correct that the concrete pad is there for water quality and erosion related reasons. That might mean that it needs to be maintained, but there's no immediate danger associated with age-related deterioration of the surface.
AFAIK, the water level is not actively controlled. If the rivers are linked to the seven seas lagoon, they're all kept at the same water level by gravity. Small leaks in the river system are not going to drain the seven seas lagoon, but they may impact erosion of the surface and water-quality.
I can't imagine any significant engineering reason why Disney would be motivated to replace the RoA. I can imagine them not wanting to pay the continued costs of basic maintenance but there's no safety issues here.