The current logic of Windows installer is that - whenever it finds an existing Windows service with corresponding mysqld.exe file version less than the version to be installed, it suggests an upgrade in the post installation step.
This does not really work since 10.0 and 5.7 as MariaDB and MySQL versions have diverged, thus 10.0 to 5.7 would
- be a downgrade, or crossgrade for which this tool was not designed
- won't work anyway, because an attempts to start MariaDB's mysqld.exe with 5.7 datadir fails (see
The user experience of trying to "upgrade" 5.7 to 10.0 during installation is that the upgrade wizard https://mariadb.com/kb/en/mariadb/upgrading-mariadb-on-windows/ fails without any useful message (trying to bring the database with mariadb's mysqld.exe).
This needs to be fixed - MariaDB MSI installer should just ignore anything that is between 5.7 and 10.0, and not offer any service upgrades .