What it should be is 2 x.5 route limit on any given plane. Not only 0.5, any x.5 route result in a one way flight, and the most a plane can have is two without teleporting.
Actually, an easier fix would be to allow 0.5 flights and lift the ban. Reduce 0.5 flight's cash advantage over 1.0 by 50-75% of current level, and call it a day. It'd be realistic as well.
In rl, on many routes people will NOT fly your airline at all if there's no daily roundtrip service, on routes such as BOS-NYC, NYC-WAS, NYC-CHI, CHI-MSP, etc. They simply want to go home after work is done.