You may be underestimating SQLite here. It has grown a LOT of features over the past decade, many of which were directly inspired by PostgreSQL.

Features you may have missed:

- CTEs - the WITH statement works great. And recursive CTEs, which can do mandelbrot fractals! https://www.sqlite.org/lang_with.html

- Surprisingly good built-in full text search: https://www.sqlite.org/fts5.html

- Functions for directly querying JSON data in columns: https://www.sqlite.org/json1.html

- Extremely comprehensive geospatial capabilities thanks to the SpatiaLite extension - this has a huge amount of functionality, which I think is better than MySQL though not yet as good as PostGIS: https://www.gaia-gis.it/fossil/libspatialite/index

I'm not saying it's as "good" as PostgreSQL, but I don't thank your argument that PostgreSQL and MySQL implement a substantially larger portion of SQL holds up particularly well.