News Microsoft persuades Mail and Calendar users to migrate to Outlook — Mail and Calendar app will cease to work after December 31

I had the new Outlook installed briefly. The first time I clicked on an item in my inbox and it turned out to not be mail at all, but an advertising link, I switched back to Thunderbird.

No, not junk mail, clicking on an item in my inbox launched a browser link, there was no mail item at all. I was furious.
 
On Windows 10, I was pushed to move my Calendars to Outlook. I agreed.

Firstly, Outlook did not adopt any of my calendars and removed my ability to launch Calendar to be able to see my calendars that way. Whenever I would try to launch Calendar, it would launch Outlook instead.

Secondly, when I did managed to reinstate Calendar on my Windows 10, all my calendars were gone.
 
Microsoft has warned its users that it will eventually have its Mail & Calendar app users switch to the open-source option for better security, but switching to a cloud-based app feels forced.

Which open-source option is Microsoft switching its users to?