Ibjoel has already said that 100 countries badge *is* coming (which is the point I came to this thread to make but has now been asked many times and replied to three times)
I think it is necessary as I can honestly claim 50 countries, but I am just a fairly lightweight holiday traveller. Many of the great and good here are well north of 100 and it doesn't seem right that I have the same badge. It does seem to get exponentially harder to tick places off the list once you start getting above 125, so I can see a point to having those higher badges, but you have to remember what they are for. They are not there for a competition to show off for the top people, they are there to make people feel good about what *they* have achieved so they engage with the site more and generate additional advertising revenue. So there needs to be a large cohort with the top badges. I presume this is also why the other badges are as they are. They are there because it's what the advertisers think is useful for their demographic and for them.
A lot of people never leave their own country which will skew the global travel statistics and some geographies make totting up the early badges almost trivial. So 10 countries might seem like a lot to someone in Kansas, but someone from The Netherlands would probably wonder why you didn't get out more. But as pointed out, here on FT there are probably enough with 100+ to make it a useful badge.
On the flip side, x million milers seems to be mostly a US thing. Because non-US airlines tend not to give rewards based on total mileage I don't think people elsewhere count it as often.