So...before this thread gets locked...
I'm going to look at this from a buisness perspective, not looking at it the way I'd like to see it for the sake of my flying career. If we go back to the first post in this thread about why should we have 5 ERJs doing a flight that 2 MD-80s can do while carrying the same ammount of people, theres a good reason behind it. There are pretty much TWO types of travelers. Buisness travelers who travel at least once or twice a month, or more. Then there are leisure travelers who travel once or twice a year, maybe a little more or less. I'd say "Buisness travelers" account for at least 70-75% of the flying public each day, if not more. Meaning "Leisure travelers" account for only 25-30% of the people who step on a plane each day(other than maybe the holidays and spring/winter break). The two biggest differences between these groups? One picks flights to get from point A to point B most direct and at the most convienient time, the other picks flights that get them from point A to point B the cheapest, even if it means stopping in points C and D.
Here is an example. Right now Skywest has 5 daily CRJ-200/700 flights from SFO-BUR. Lets say every flight went out full, the 4 CRJ-200s would carry 200 pax while the daily CR7 would carry 66, thats 266 pax. United's A319s carry 118 people or so. Lets say they switch the flight to 2 daily A319s, one in the morning and one in the evening, to carry 236 passangers a day. What we would probably see happen is both flights take a huge hit and maybe go out half empty everyday. United may now sell 110 seats a day when they were selling 266 with the RJs. Why? The buisness travelers now have only 2 ways to get to Burbank, take it or leave it. If that schedual works for them on that day, great, if it doesn't, they're going to SNA or LAX on Southwest instead or driving to OAK or SJC and getting to BUR that way. The leisure travelers don't care, they can still make it to SFO from MFR, RNO, SMF, ect in time to catch the flight, whats a longer layover if its cheaper than flying MFR-LAX nonstop on Horizion? But as I said before, the leisure travelers aren't (usually) what fills a plane. And reducing the frequency from 5 flights to 2, regardless of the size of the aircraft, isn't going to appeal to MOST flyers.
And when you DO have 5 flights a day, you probably won't fill 5 A319s on SFO-BUR, but an RJ will fill up. Infact, United used to have 5 or 6 flights a day on the route with 737s back in the "Shuttle by United" days, and they figured this out. It makes sense and it makes money.
Like I said, I'm not supporting regionals taking all the mainline routes, I'm just shedding some light as to why this is not an ideal solution in most cases.