It really doesn't matter much anymore. MariaDB doesn't have the significance it once had. Worst case scenario if it were to disappear people would switch to Postgres and that's that.
Huh? Postgresql is not mysql compatible. Mariadb is very popular in a ton of businesses around the world as a not stupidly expensive sql database with great support.
Neither is MariaDB, necessarily... if you think you can simply convert MariaDB databases to MySQL and pick up right where you left off you may be surprised. Depends on a variety of factors including which version you use, what features, the code you've built around it etc.
I would not be surprised. I've done it many times including complicated setups with different databases as replica slaves.
I'm now seeing a lot of new projects that don't care much about DB backend since the library they use to wrap sql calls obscures all that stuff anyway, but I promise you mysql to Maria is a much more common and straightforward transition than to postgresql.
If they were gonna use MySQL I have to imagine they would have used it already. If they're using MariaDB they're probably using features that aren't in MySQL – either that or they want to avoid Oracle. Also, migrating Maria databases and codebase to MySQL is not as straightforward as you might think.
Anyway... MariaDB is not dead yet, no need to speculate that far ahead.