Perhaps in the US, but throughout the world most FF programs take better care of the high revenue passengers. Consider CX and SQ, until SARS the most consistently profitable airlines in the world. Full Y counts for nothing toward the three highest levels in SQ's FF program (PPS, PPS Solitaire, and Lifetime Solitaire). The CX program is similar. Except to satisfy an operational requirement, CX and SQ give out free upgrades as often as Jesua bin Josef returns to life.

People don't buy what they know they can have for free. I used to regularly buy $300 RT SFO-LHR tickets and sit in F 60% of the time and C 40% of the time. Then UA made double upgrades virtually impossible so I started buying C tickets. Smart move on UA's part, though I can't say I was happy about it.