Acuña’s Bat Flip
NL Rookie of the Year
Pache is FAR more "available" in trades than Riley. Ender is signed through 2022 when Waters would be ready, and some services already are ranking Waters higher than Pache. There's not another 3B in the system behind Riley until you get to C. J. Alexander, and he's likely not going to be good enough defensively to stick over there.
If you trade Riley, you may well have no choice other than to spend big money on Donaldson next winter - Arenado may be close to an extension to stay in Colorado, and the Gnats will probably extend Rendon (possibly even if they bring Harper back, but almost definitely if they don't). That leaves you with Camargo (who AA apparently doesn't believe in as an everyday 3B) and Donaldson next winter if Riley's in Miami (or elsewhere).
That could end up being the equivalent of trading Mookie Betts for two years of Tucker Barnhart in the long term. We have plenty of time and different trade scenarios to worry about the future of 3B beyond this season.