But the top prize is one million miles, which would get you lifetime status. Delta and United don't (yet) have spend requirements for lifetime status, AFAIK.

Conferring lifetime status isn't necessarily even that expensive to the airline--you only get the benefits after you give the airline your money for tickets. It also creates an incentives to buy tickets on one particular airline, even when they might be slightly more expensive (which is the whole purpose of frequent flier CRM programs).

Yep, exactly. If they were 1mil PQM or 1mil BIS you'd hit million miler and get gold for life. Gold, which only kicks in when you actually fly (like you pointed out).