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.

You don't even need SQLite for that. Something like PicoCMS with markdown files is more than enough. If you want a fronted for the owner, that's another story, but if you're the one mantaining the sites, WordPress is an overkill 99% of the time.

Pleasantly surprised to hear PicoCMS mentioned here, since it's not typically the most common PHP markdown CMS of choice folks like to mention.

I was a casual developer (i.e. not for work, just for personal) deep into the PicoCMS ecosystem for a couple years, a few years ago. I both started a site and helped a family convert an old static site to PicoCMS and really had no complaints. Re: frontend for the owner, I started with Pico Admin and made a bunch of modifications to it (including an image uploader) and the non-tech owner has no complaints and it's been working well since.

Nowadays for my own blog I'm into the whole SSG/JAM trend, but I'd still run a PicoCMS site any time, if the use case is right.

I've been reviewing CMSs for some time, and PicoCMS is the best in terms of simplicity, hands-off and easiness.

I looked at using SQLite with Wordpress for sites I maintain. In theory Wordpress core is quite DB-agnostic and according to random blogs MySQL could work. But plugins use so much MySQL-specific features that it was hopeless.