Had this been complete, I would've completely agreed with you. But it wasn't. (and again, I'm not saying United wasn't prudent - surely they were.). And I do love Pentagram's work - it really was a "complete" branding.

But being objective for a moment - you had one company with a mish-mash of logos, signage, liveries and branding. And one which was ruthlessly consistent, for better or worse.

They chose consistent. And I don't think anyone could ever convince me that cost wasn't the overriding factor.

One fleet (CO) had planes with recent paint, all in good condition.
The other (UA) had at least 3 different liveries, all in various states of disrepair. Some worse than others, but as a whole, the fleet was looking very tired.

If they had chosen, say, Rising Blue -- both the UA and CO fleets would've needed a full tip-to-tail repaint.
By choosing the CO logo, they could repaint the UA fleet, and just change the lettering on the CO fleet. Saving a pile of cash. And keeping those CO planes out of service a lot less. (Winding the clock back, CO was known for a very very high fleet utilization ...)

Same with signage and gates and such - we see lots of examples where the word "Continental" was scraped off, and United put in place, but the rest of the branding remains the same.

I'm convinced it was primarily a cost move. Keeping the CO logo and changing the lettering was an immediate-cost-savings move. And - let's be frank - would picking the tulip increase sales? I doubt it.