Fare increases could be route specific. An airline could choose to raise fares on routes due to consolidation that have little to no competition. Thus the 21 day rule would have no impact.

What I see as the major problem with airline consolidation is the control at certain hubs. e.g. Atlanta for DL, DFW for AA, MSP for NW, ORD for UA etc. When the FTC looks at these ariline mergers they really need to get gate concessions to open up other airlines coming in to compete. There are only so many slots allocated at an airport. Once an airport is at capacity for landings and takeoffs that's it. ATA or SW are not going to be able to get slots and the dominant airline at that hub is going to have a monopoly on certain routes. Thus you see fares of $500 for 1 hr flt from ORD to MSP. Look at LAX as an example. There are no dominant carriers there and flts from the east coast into LAX are reasonable because of the competition in and out of LAX.

In the end though, it all comes down to supply and demand. If the country does go into recession and the American public stops flying, regardless of who controls what routes, the airlines will be forced to reduce fares to increase demand. Either that or start reducing capacity. I don't see that being the trend though.