I’ve recently noticed that the game tends to over-inflate the number of per flight PAX (see route research). This means that tiny airport pairs (with only 10-25k total PAX) easily support multiple daily A320/B737 services at 100% awareness. This problem is compounded when you are servicing a tiny airport from your main hub. Even if your hub city isn’t that big, you can probably fill even a 757 or A321 multiple times a day to some tiny airport that in real life maybe sees 1 CRJ flight each day?
For me, this takes a little bit of fun out of matching the right aircraft to the route. There’s little use for E-Jets, CRJs, ERJs, or A220s (let alone Caravans and Jetstreams) when every airport pair in the game seems to support A320 service.
I think it would be more challenging, and hence more fun to reduce the number of PAX (however this is calculated) to make A320/B737 service impracticable on some routes to tiny airports, making smaller aircraft the best choice. Right now, the only real route constraint is runway length, PAX numbers seem to have basically no impact at all.