Calendar is not going away, calendar events are not being touched, this has nothing to do with alerts for calendar events, and this does not require you to use a new/separate app.
Google has simply had different things called both "reminders" and "tasks" across apps, for legacy reasons. For a long time there's been a "reminder" object in Calendar (different from events), and then a "task" object got added as well (that shares data with the Tasks app). There's no good UX reason for these to be separate types of objects, so now "reminders" and "tasks" are being consolidated into just "tasks". But they're staying in Calendar. (And can also be edited in the Tasks app which has been around for years already.)
This is good. It's simplifying.
(As to why they're now splitting off reminders created in Google Keep rather than integrating them too, I don't know for sure. But I've never understood the use case of reminders in Keep at all, so I wouldn't be surprised if they deprecate that feature entirely. Keep reminders came before Tasks existed at all, but now you should just use Tasks.)