I don't know how much more profit can be squeezed out of a typical Subway store. While a few folks who own multiple stores do relatively OK, franchisees with only one or two stores make a pittance. Demands on franchisees are regarded as the most ridiculously oppressive of any food franchise.
As to sandwiches, Pittsburgh is a good to great sandwich town with a notable exception - no Jewish delis. And very few true delis of any kind. Most supermarkets don't even make sandwiches at the deli counter. That said, a few decent Reubens can be had at sit down restaurants/bars. But pastrami is pretty much non-existent here, which is a bit surprising as if there's one city that would readily adopt the LA burger topped with pastrami, it's here.
'Depends. Owner operated can make good money. Absentee owners aren't going to do so well. This is likely true throughout the industry.