It’s pretty much the definition of fixed costs versus variable costs.

You spend the same amount on R&D whether you have one hobbyist user or 90% market share.

Yes. R&D is guaranteed to fall as a percentage of costs eventually. The only question is when, and there is also a question of who is still solvent when that time comes. It is competition and an innovation race that keeps it so high, and it won't stay so high forever. Either rising revenues or falling competition will bring R&D costs down as a percentage of revenue at some point.