This is very true. I've been Plat, Gold and currently Silver. I've talked about moving from Delta to UA for years. The biggest reason I haven't is I also fly a lot to Vegas and average 40-50% upgrades. Usually upgraded there (last Friday) and sometimes back like last Tuesday. Upgrades are tougher on Friday's, Sunday's and Monday's or going through Atl, JFK or Lax. I only use Sky Peso's when I find a deal or for relocating. So as long as I keep getting upgraded I'll stay with Delta. The rollover miles may also help you to Silver 1 year and Gold the next like me. I've been going back and forth for 3 years. I'll be Gold no later than Sept. Then it will last until 2/28/2015.
A good question is who's FF program will they go with USA's or AA's? If it's USA's stay with them and see if you're happy.