Any ideas at what point string hopping becomes unviable for alternate picking?
The way it’s defined now, all the time.
String hopping is not a motion, it means you use the same muscles in both directions to escape the strings.
So it’s kindof the definition of inefficient.
Anyway it’s pretty common that stringhopping is used at slower speeds, so I’d say it’s kindof a personal decision.
If you fell fine in certain passages with stringhopping and there’s no problems with stamina and the transition between the different motions, that’s just fine.