Delta/AA/B6/UA fly virtually hourly shuttle flights from NYC to BOS, DC, ORD, LAX because business travellers want choice of departure and arrival - not because they don't have bigger planes.
Taking 10 daily flights and turning it into 5 isn't optimal from a passenger perspective on business routes where it means I have 2 hours to kill because my meeting is timed wrong.
It also means if my meeting ends early, I have a chance on hopping on an earlier flight.
A high frequency mainline flight + a low frequency regional leg = You can get a short/optimal connection time with at least one of those mainline flight choices.
But if both flights are low frequency, you wind up with a lot of connections hitting unappealing layover times that drive business to the competitor with a shorter layover.
[1] Hopefully in 2050 we'll have rediscovered how to build railways in the USA and actually have HSR from NYC to BOS making this example a bad one.
It's disheartening both that Gov Newsom wants to kill it and how ecstatic people are about that. The 'can't do' culture is very strong in the US.
Operating planes designed for spanning the globe (777, 787, 350, etc) over shorter distances (<4,000 miles) just to get the extra seats becomes much less economical due to the weight of those planes and the wasted space on cargo capacity that can't be filled on the shorter routes.