Depends on the scenario.
If the SSD is just a buffer for a harddrive, using SATA makes no sense at all. Users wanting greater capacity will exchange/complement the hard drive, not the fast buffer. In fact, it might just as well be soldered.
If you actually store your purchases on the SSD, then you might eventually want to replace or add a complementary drive. The easiest way is to hang your original SSD on PCI-e and simply unplug it, and plug in your new and bigger/faster.
It is only in the scenario you painted, where you store all your games, and run them, from an internal SSD, and want to complement that with an
external SSD connected via USB-C where games are also stored and run from,
and you assume that having a faster internal SSD could create a speed mismatch that could somehow create problems.
To me, that seems like an unlikely scenario. I don’t think it is the most probable hardware configuration (one large SSD to store everything in the base configuration), and even if it is, I wouldn’t assume that having a slow internal SSD is a good idea anyway. Saddling all systems with a needlessly slow SSD on the assumption that the developers or OS cannot handle the user adding slower external drives seems like dubious design decision when it is actually easily solvable (if indeed the design follows this approach in the first place), by just designating part of the fast internal storage as scratch space for the active game.
(Personally, I think having 128GB of really fast SSD acting as a buffer for games stored on a large, cheap harddrive is a reasonable approach. Cost matters. Speed matters. This approach adresses both in a reasonably cost effective way.)