Someone exceptionally good at something is likely to spend almost all of their free time on that something. If so they won't have time to routinely blog about it.

Yes, this is indeed so. Still, it may be possible for them to blog at least some more without very much cost to themselves.

Almost everyone who blogs a lot eventually posts something very stupid. If you strike someone off your list of elite thinkers when they say something really dumb, you'll end up with a list dominated by non-bloggers. (See also this old taw comment.)

• This may be a reason not to blog a lot, but not a reason not to blog at all.
• It's awfully silly to strike someone off of your elite thinkers for things that they say in a few blog posts, especially if they redact their positions later on. But the issue that you describe could be a deterrent in practice.
• I think the world would be a better place if people worried less about their reputations.