Remember the BAe125? The latest model was called BAe125-1000, I guess Boeing would go the same way, IF there will ever be an additional 737 model.
No, it's not true that the 737 "has had its day". With new flightdeck and engines, the 737NG surely is a good aircraft. The only disadvantage (imho) is the narrow fuselage, in comparison to the A320 fuselage. It's only a few centimeters, but passengers seem to value those few cm's. So I think for the next 100-190 seater, Boeing should develop a new aircraft with wider fuselage for 150-200 passengers (B787, for example), plus new larger versions of the 717 with standard 717 fuselage for 120-150 passengers.
Aircraft families are a good thing. But if that means that we get a 100-seat A320 (oh pardonnez-moi, A318
), or a 737 large enough for 200+ passengers but emergency exits for 189 passengers only, I start wondering. Well, airlines get what they asked for..