Aside: why, oh why, did 3 mysql forks appear when MySQL AB was bought? Couldn't they coalesce around one alternative?
Ordinarily, I'm not against a little healthy competition. I think forks and trying new ideas is great. But the timing just makes no sense to me from any rational point of view.
"I suspect keeping up with it does not require as much effort as it did getting to where it is now... maybe... it will continue evolving."
True, but I'd hate to see the bar set that low.
Percona Server exists because Percona has a great deal of deep hands on knowledge about MySQL tuning and internals, and has a live-or-die commitment to making MySQL work well, and taking patches from it's user community.
Maria and Percona server advance more or less together, and routinely swap patches back and forth.
DrizzleDB exists because some of the MySQL developers decided that Doing It Right was more important than maintaining old targets and maintaining backwards compatibility.
There is plenty of room in the MySQL user and developer ecology for both approaches. If you need MySQL, and you don't trust Oracle, or don't want to pay them, just swap in MariaDB or Percona Server, they are drop-in replacements. In fact, you should drop them in away, they have more bugs fixed and are more performant than stock Oracle MySQL.
They had different goals. MariaDB was to maybe make another billion and Drizzle was to be more parallel and remove all the Windows specific code, IIRC.