Ok, so I found this:
https://manual.calibre-ebook.com/faq...-of-mobi-files
That answers a part of my question. Also, the support for mobi is not taken rfom Kindles themselves, just from the e-mail service, so even when the e-mail service will stop supporting mobi, it will be possible to load them through USB.
And for some reason, Calibre cannot produce POBI files, it can only read them, so maybe using those instead would be major work.
Edit: I*wrote the above in the meantime when Kovid replied.