What is the future of staple Mac apps like mail, calendar and reminders?
Nobody knows and I certainly haven't seen any rumours that they're going away - shipping a modern operating system without that functionality would be a "courageous" decision.
What
is possible is that some of the regular apps will be replaced with "project catalyst" versions - see
https://www.macrumors.com/2019/06/04/what-is-project-catalyst-marzipan-mac/ - that are shared with iPadOS. Bear in mind that News, Stocks, Home and Voice Memos apps on MacOS are
already catalyst apps - and I'd venture that any new or significantly upgraded "desk accessory"-style app - that doesn't have drastically different functionality on iPad and Mac - is likely to follow suit, because it avoids duplication of effort.
I think for something like calendar, the user wouldn't necessarily notice the difference (it's for Apple to make a good or bad job of re-designing it). Also, with iPadOS, Apple is trying to distinguish iPad from the iPhone and push it as a serious laptop alternative so,
hopefully, this would be a case of beefing-up iPad apps rather than dumbing-down Mac ones.
Of course, it's 2020 and "worse" is the new "better". Everybody hates Mail but I'm sure that, if it changes, everybody will be begging for the old Mail back... (*cough* iTunes *cough*).
This is, of course, pure speculation.