Actually, if we had just three legacies, I imagine there would be tremendous overlap. Each one with a different route network out of each station, but lots of redundancy.
Example, my weekly commute SFO-DAY. On UA, I go SFO-DEN-DAY or SFO-ORD-DAY. Between those two routings, there are around 7 total itins on UA that I could book on any one day to get me there.
On NGA, I can go SFO-MSP-DAY, SFO-DTW-DAY, SFO-CVG-DAY, SFO-SLC-DAY, SFO-ATL-DAY, SFO-MEM-DAY. That's a total of 6 routings, with a few of those having a couple acceptable itins. With some consolidation to the route network, I don't have any trouble believing that NGA will be able to get me out of a UA hub and where I need to go with approximately the same convenience as UA could.
(None of this holds true for hub-to-hub flights.)
Not sure what I'm expecting, just trying to remember that it was the days of a regulated airline industry that cooked up such perks as airline lounges, mileage programs, ground transfers, meal allowances and more.