The query planners have diverged between MySQL and MariaDB; which one are you talking about? Both have received improvements in recent years, so the specific version is relevant as well. And at least both systems have supported index hints
MySQL - we use the index hints 100% of the time because the query planner (still) doesn't work
The real-world relevance is minor, especially since companies using external write-through caches are already making much worse inherent trade-offs in that area
I disagree - everyone knows that the cache value can be stale, and to be wary of it if your use case requires correctness. In my experience people tend to expect correct, up to date data from their database - which is why they usually have to opt in to read-replicas
Do you have some novel academic solution for this that you can propose?
MySQL doesn't support synchronous replication - so semi sync is the next best thing. Then you wrestle with it to make it not crazy (e.g. - only count a tx as acked if it was acked by a replica that has all the other txes). Its not rocket science - maybe the big fanfare about synchronous replication made sense when a disk write was incredibly slow, but adding 2 ms of network latency and 5ms to persist it on another machine in order to not lose user data seems like a pretty straightforward choice to me. And if write performance matters that much to you - you shouldn't be using a dinosaur like MySQL anyways