767 sales to U.S. passenger fleets were dead long before the CO/UA merger. CO and NW had ordered 787s; US and NW had A330s on-property; DL was focused on ULH, getting 77Ls. AA and UA had big 777 fleets.
But UA had a First cabin way before the merger, ergo, they didn't need a large fleet of smaller capacity aircraft given they didn't really fly to any smaller "non-First justified" markets. Had they not had a First cabin, I was postulating they might have invested in more 767s.
I'm not sure I follow your train of thought and how UA's first class cabin was a factor. If we rewind the clock back to 1990 through 1995 how many secondary European markets did AA and DL serve?
I agree UA only enter the secondary European market as a result of the merger with CO but without knowing how much of a factor the secondary market was in AA and DL's network in the early 90s it is hard to see the connection between the secondary European market and UA's early 767-300ER order.
I'm trying to recall from memory and this changed over the years. These are the cities I would not categorize as justifying a First Class market (my own interpretation of course)AA
ORY (RDU,BNA,MIA, DFW)
a bit harder with the acquisition of PA's Europe network and the connecting hub in FRA. DL did have a First Class back then too.
but they didn't fly to LHR
But they did inherit many secondary and seasonal markets off the Med (LIS, BCN, ATH, FCO) as well as Eastern and Northern Europe (OTP, BUD, PRG, WAW, HEL, ARN, FBU) that were never strong First markets.
United on the other hand by 2000, was only flying to LHR, CDG, FRA, AMS, BRU, MXP, MUC and DUS and only from NYC, BOS, ORD, LAX, SFO, and IAD. Not from BNA, RDU, MSP, ATL, or CVG.
The 767s really needed to maximize real estate to increase RASM. Southern Europe markets are bumper fares in the height of summer, but United would get slaughtered with a 767 with low cabin density. Just wasn't their market, and therefore didn't need the extra 767s.