I like these kinds of blog posts. Not because he's ditching mongo, I like the detail around his thought process because there's an over-abundance of articles about "large company does X" but less of the hobbyist version of the same.
Sounds reasonable and not hype train driven
As opposed to?
You can use an SQL db engine as a kv-store, but you can probably make a more efficient kv-store if you do not have the SQL constraints.
(Oth SQL engines are pretty efficient for lots of use cases these days)
Another example where SQL may not be the right choice is time series or graph DBs.