Flying a 19 seat airplane in the US under part 121 rules tends not to be cost effective... Twin Otters, as much as I love them, don't qualify anyway.
Yeah I know they aren't Part 25 or Commuter category certified. there are waivers, in fact one operator (whose name escapes me at the moment, I apologize) is currently operating Twin Otters under Part 121. The Beech 1900D seems to be a good 19-seat airliner that's still in fairly heavy use even in the US. Great Lakes operates a fleet of 32 of them.
But generally I do agree the 19-seat is the bottom of the airliner world and thus difficult to make profitable. However if you can only sell 15 seats on a leg on average then a 19-seat option is certainly more cost effective than a 37-seat or 78-seat option.
My point was that how profitable an aircraft can be on a route is linked with how many seats can be expected to be sold on that route. Overarching comments like "there is no market for a 30-seat airliner" is flawed by the assumption that markets can yield growth beyond 20-30 passengers per leg. There are many markets worldwide that can't grow beyond that and still require air service.