You can do everything with the Plesk command line tools that you can do with the GUI. You can for sure create backups that are stored in an external FTP repository. The only prerequisite is that your storage space needs to have an FTP access.
For example you can simply lease a web space or server or cloud storage space anywhere, as long as it has an FTP access, and then store your backups to that storage space. However, using special services like Dropbox, GDrive or whatsoever might not necessarily have an FTP access point, so for these you may need a different approach. That's why there are extensions for such services.
Here is a question for you which answer will help me to understand user requirments better: My experience is that users always want something that they do not have. It does not really matter how much functionality a software has, there is always something that users will want in addition to it. They have all opportunities, yet they find something additional that they want to do. Why not simply back up into an FTP account somewhere? And why not simply create one backup and then doing a copy of that if you absolutely need a second copy of the backup? If Plesk had Dropbox, GDrive integrated as an option, users would probably ask why it does not have Amazon AWS E3 direct storage space access implemented or some other option that is not built-in. If it also had Dropbox, GDrive, Amazon AWS support, they'd ask why JohnDoeBackupServices are missing ... In my company we've never needed anything beyond simply FTP/FTPS, and I guarantee we have a really huge demand for backups and huge backup spaces with daily backups in the high Terabyte range, and there has never een any need for anything beyond that. So for normal use why not start with what's there, and if it turns out that his is not enough you can always think about having someone develop a Plesk extension for a special service that provides even more functionality?