I don't have any data on this, but I do know that a significant (if not majority) of Wordpress sites now aren't even blogs. They are small websites for things like restaurants, hair salons, construction companies, photographers, and personal portfolios. Especially since most of these sites are also running on $3/mo shared hosts, switching to SQLite would be a substantial improvement.

Most these sites are running a homepage, an about page, a contact page, and maybe one or two misc pages. They might have a blog that has two blog posts on it from nine years ago. But that is really it. MySQL is really overkill considering the scenario. SQLite's big "limitation" is non-concurrent writes. But this is rarely a problem with most Wordpress sites because they are single-author and they aren't updated very often. SQLite can handle plenty of reads to support even heavily trafficked websites.

Not to mention, SQLite's greatest advantage is portability. A single file contains your entire database. A non-technical user could transfer hosts or backup their data by copying their database file like it was a photo or an excel document. That's pretty incredible when you think about it.