I don't see how storing the mails in a database will improve the situation. You will still have to update 1000s of records when you mark 1000 messages as seen/u

I don't see how storing the mails in a database will improve the situation. You will still have to update 1000s of records when you mark 1000 messages as seen/unseen or move them into the trash. And the DB will then write the changes to a filesystem. One of Maildir's design goals was to make it safer to use on networked file systems such as NFS (compared to mbox).