1K here (12th year now). Fly about 200K miles / year, about $20K / yr expenditure. ENgineering consultant FWIW.
One time I was upgraded to First Class (domestic, US) and I sat next to the guy (at that time, literally a guy, a man) who essentially runs/administers the GS program.
He was extremely friendly, and as the drinks & flight continued, I pulled a /small/ amount of info from him about GS.
BASICALLY, it comes down to profit. Every year the bar changes, but if you are something like the 0.1% most profitable customer, you are in GS. Every year the amount varies, but typically you need to buy $50K - $75K of high-profit airfare (read: full class airfare, usually highest class of service, often international)
If you saw me (small business owner, consultant) you'd think I'm either a schlub or an athlete, but not a "CEO type". I am CEO of a one-person company. I do very well, but I don't work for Google / Bain / McKinsey / etc. So I book inexpensive airfares (it's my company, my money) and I accept that there are employees who just travel a lot and get treated like royalty.
I would love to have others jump in and tell me this next statement is flat-out wrong, but IMO the GS folks aren't entrepreneurs and small business owners, they work for BigCo and are treated like royalty. Not jealous at all, from my informal survey the divorce rate of GS is way higher than average. Also many GS folks on the "fringe" are stressed out b/c once they've had it, they don't know exactly what they did to get it, and what it will take next year.
I sleep well at night.