If you in the dialog "Really migrate to MariaDB?" choose the option "No" it will bail out from dpkg and abort the installation. This is by design - you are not supposed to get MariaDB installed if you choose "No".
This method of aborting has been around in MySQL packaging for a long time. For example if you had MySQL 5.1 installed and then installed MySQL 5.0 you would see a similar screen asking for confirmation and if you selected "No" then the dpkg process would be aborted.
If you in the dialog "Really migrate to MariaDB?" choose the option "No" it will bail out from dpkg and abort the installation. This is by design - you are not supposed to get MariaDB installed if you choose "No".
This method of aborting has been around in MySQL packaging for a long time. For example if you had MySQL 5.1 installed and then installed MySQL 5.0 you would see a similar screen asking for confirmation and if you selected "No" then the dpkg process would be aborted.