There is so much variation in the dollars vs. miles thresholds I can only conclude one thing: this is UA's way of sniffing out FlyerTalk account names for those of us who like to complain.
With that said, my offer made sense based on my travel last year, but is unobtanium for my travel this year.
Milestone 1: spend $20,500 and earn 39,000 bonus award miles
Milestone 2: spend $23,000 total and earn 58,000 more award miles
Milestone 3: spend $28,000 total and earn 96,000 more award miles