No stats to support this, but I seem to remember reading here in the past
that out of all the individuals who get a student license, 90% drop out.
Out of those that get the PPL, 90% stop flying after 5 years.

I remember reading that too, I guess its possible. If 200,000 got
their PPLs this year in the US, then it would mean there were about 2
million student pilots, not outside the realms of plausibility.
I think there was another stat about how some pilots never flew again
after passing their checkrides.