Nootbaar, Gorman and Yepez are all interesting internal options. So is Sosa, based on his limited work as a starter (.294 batting average, .795 OPS). But, yes, you can only speculate on how well the prospects would perform at the MLB level over the long haul. The Cardinals have three options overall on offense:
1) Add both a proven shortstop and a proven DH/OF from the outside and tell the young guys to wait. That could lead to another Luke Voit/Randy Arozarena-type trade trade down the road when a blocked prospect gets moved to fill another need.
2) Add a proven shortstop and let multiple prospects fight for at bats at DH, fourth outfielder and second base.
3) Run with the internal replacements to start the spring and then make changes on the fly as needed, as the Cardinals did this season (belatedly) with their starting rotation and bullpen.