Yeah, but how much of that was due to forking and how much of that was folks realising that PostgreSQL is much better than MySQL for most metrics of better?
Yes, but Postgres had lots of technical advantages of MySQL for years, and still was less popular. For most uses people would decide "MySQL is good enough feature-wise for me. It's so popular it's probably safest ecosystem-wise"
It was the MariaDB fork that shook people up. Now suddenly the decision became "should I bet on one of these forks, or should I skip that drama and use Postgres which seems like a nice stable project" The fork is really what changed the game.
The other thing to consider is that for me to think that forking makes the future of the project questionable, you'd have to show that it's likely that usage(original project) < usage(upstream) + usage(fork). In other words, I'd be interested to know the combined usage of MariaDB and MySQL and see if you could actually predict with confidence that it would significantly lower than original MySQL usage. Not just that one of the forks didn't have as many people.
It was the MariaDB fork that shook people up. Now suddenly the decision became "should I bet on one of these forks, or should I skip that drama and use Postgres which seems like a nice stable project" The fork is really what changed the game.