It's an interesting idea, for sure, but aren't many Wordpress sites hosted on shared hosting solutions that provide a MySQL/MariaDB database free of charge?
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Yes, but that doesn't mean we shouldn't have sqlite. Many people also run WP setups on small VPS and don't really want/need mysql running and wasting ram all the time. Having sqlite also opens the door to database git versioning and making local dev easier. While none of this will work at scale they'll still provide immense value for a lot of people.
Most WP websites are read-only and they could run just fine on sqlite. Even if you've a large number of reads, WP people are used to cache everything as static HTML files so it won't be that bad either. For what's worth sqlite is also getting faster/better at concurrency (WAL and whatnot) and it may even be "about 30% faster than mysql" in some cases.