Third party content designed for 5e is one thing, but even if TotV is still compatible at the start, that doesn't guarantee it will remain so over time as it grows into more and more of its own thing. See Pathfinder going from basically an update to 3.5e to its own distinct system.
I think supporting multiple full game systems on Beyond at the same time is unlikely, unless they're aiming to turn it into a system-neutral platform. And I don't think anyone wants WotC/Beyond becoming the Valve/Steam for TTRPG digital tools after their OGL shenanigans.