It's way way way way way way less than 1%. Look at it this way: To get 2MM BIS miles in 20 years means you have to fly at least 100,000 a year annually. That puts you at a high elite level annually. But it's a fairly small number of people in any given year who even get to that level. And then how many of them keep up for 20 years?

These statistics (of yearly status level) are probably easier to find that the million miler statistics.

(Of the general population it's below miniscule, since most of the general population does not even fly enough to ever get status anywhere. So all that even matters is the percentage of one-time-ever elites who evenually get lifetime status and even that is tiny because so many people are only lowest level elite plus so many people are not even elite consistently every year.)

Also, at some airlines (dunno if UA/CO is one of them through either the UA or CO branch), it was easier to earn MM in the past than today, but once you earn it, you keep it. AA was the last one (until Dec 2011 you could earn AA MM from all miles from all sources, not just BIS), but it wasn't the only one to count non-BIS toward MM, if you go back far enough in time.

So while going foward MM is BIS only, counting how many people have MM status already is a different story, because of historical changes to MM earning.

(And I'm a perfect example of this. As someone who basically never flies for work, and didn't start flying much at all until in my late 40s, there's no way I'd ever have gotten MM anywhere if I hadn't learned on FT years ago about all AA miles from all sources back then counting toward MM, and started working on that, with a sense of "urgency" which turned out to be warranted, since I made 2MM "just in the nick of time" in 2011.)