Hall of Fame
I think in a very high population metro area this is a workable possibility (see ALTA). There are participation numbers sufficient to make those leagues workable.
I think in the rest of the country it is non-supportable. Some areas cannot even support the levels USTA has right now ... how would they do more?
You nailed it. Lot's of places can't even fill a 4.5 level. Women's teams can be especially thin toward the upper levels.