Obviously, you know nothing of airline capacity planning.
To oversimplify things ... what's an airline to do for the inevitable slow days ....
Keep 200 spare Regional Jets around to substitute for the MD-88/A320/737/etc on the slow days ?
Oh .. gotta keep 200 spare Regional Jet Captains and 200 RJ First Officers around also. The mainline guys probably aren't current on the RJs.
Hmmm ... same goes for the cabin crew.

And let's not forget that the plane you're thinking Anderson should've substituted to a smaller plane actually had more than 1 assignment today.
I'm willing to bet you that somewhere along that plane's assigned routes, it probably had a load of passengers that wouldn't fit on an RJ.
So I guess that in addition to 200 spare RJs at the hub, we should also have a few Mainline spare jets at each "spoke" city. And the crews on standby to fly them.
Of course, once the spoke city gets the substitute RJ inbound, and re-subs back to a mainline aircraft, they now have a spare RJ just sitting there. Oh, and the crew that flew it in. I guess that RJ plane & crew can just sit idle at the spoke city until the day that the opposite happens ... an inbound flight sked as an RJ was uplifted to a Mainline due to a heavier than normal load .. so the spoke can re-sub the RJ back onto the routing and all will be balanced again.

Truth is ... in most cases it is cheaper for an airline to maintain the status quo with fleet assignments than to try and shuffle aircraft for load factors.
And even though he isn't directly involved with aircraft scheduling, I'm willing to bet that Anderson knows more about this concept than you do.

And of course you're completely ignoring scheduled aircraft maintenance checks. A common check, called the "B Check", has to be performed every "x" hours of flight. That works out to be about every 3 days. When an aircraft comes out of it's B Check, it is assigned to a specific routing that optimizes its use until the next B Check comes due. Specifically, its routing will have it hitting the "x" hour mark late in the evening at a station qualified and equipped to do the check. Start juggling assignments to satisfy the "arm chair airline experts" and you end up with planes needing to be pulled offline in the middle of the day and/or at stations that can't perform the checks. Now you're ferrying empty planes around for scheduled maint checks. That's even worse than flying a 150 seater on a revenue flight with only 48 tickets sold. Trust me.

Yes, airlines substitute airplanes all the time. There is the rare occasion when say an MD-88 has a light load out, and returns right away to the hub with another light load. And there's a spare RJ sitting on the ramp. And there happens to be a crew at the airport that can fly the RJ (this usually because another RJ flight had to cancel, say due to destination weather). But that's rare.
Most substitutions are due to unscheduled maintenance (something broke). The options were to either accept the high costs of the substitution or to cancel the flight. And of course, for the same reasons, cancellations are super expensive ... at a minimum, you're cancelling 2 flights .. this one and its return flight back to the hub.

Bottom line. Airline Operations are complicated puzzles. The operation isn't complete until all the puzzle pieces fit. A 150 seat airplane with 48 tickets sold is just a very small piece of the overall puzzle.

Steve