Well, I know for sure UA doesn't upgrade at the particular upgrade window if NF or NC (which roughly correspond to DL's V inventory) are 0. Also, AA only gives unlimited upgrades to ExPlat members, which requires 100k EQM annually (I may be a little off on my understandings of their program). So, to be honest, it's not quite as rosy at the other airlines as you might think.

Also, you might want to consider quality of service rendered. From what I've seen on FT, the airlines with the most generous upgrade policies (DL and US) tend to have the worse FC offerings, speaking in terms of hard product. I've repeatedly heard that AA's FC is one of the better domestic cabins--particularly in terms of food, since they don't do "free" upgrades for many people. Of course, your opinion of each airline's cabin may differ, but it might be something to think about.

I'm a DL platinum, and I like it just fine. Yes, I do get upgraded pretty consistently (of course, that may change). But I also enjoy the other benefits like SkyPriority, free award redeposits (that has saved me several hundred dollars and given me great flexibility), the baggage allowance, almost guaranteed exit rows in case my upgrade doesn't clear, and quick customer service (although sometimes you have to work a little with them).

I'm not trying to be a DL apologist. They certainly have their issues, especially with mileage redemption. But I do find a lot of things that work for me, so I continue to fly Delta. I hope you find the place that works for you, and happy travels!