This maybe makes a case for flying NW, but not CO. Even the most ardent supporters here will admit that getting upgraded on CO transcons or major hub routes does not happen very often. I like to think back to my wife, who was flying CO every week between BOS and IAH. When she started flying the route as a Gold, she was upgraded very rarely - I would say maybe 10% of the time. This is such a low rate that I'm not at all sure you can't match that using stickers, and have some more control over using those upgrades when you want, rather than when CO wants. Now, as a Plat, her upgrades increased, but to something less than 50%. So, unless our OP will garner Plat...

As for whether someone is crazy to fly UA, I really don't get this. Aside from considerations about routing or schedule or the like, I would think UA's better reward redemption is a strong incentive. Indeed, while I know it is splitting hairs, you state "but an informed flyer would save the miles for international first class trips", yet you have a much better shot of getting into a true international F cabin with UA miles than you do with CO miles (where your only option is a now-very-expensive redemption on QF).

Finally, for me, and I'm sure plenty more like me, it is plainly obvious that:

UA E+ >> CO Y-

If one flies paid F, or happens to fly a route/times with an abnormal upgrade %, then CO is a fantastic airline and program. If you fly Y and buy cheap tickets through a corporate TA, then CO is not a great airline and program.