Your calendars are tied up with your email because Microsoft Exchange decided to bring that feature to business ages ago and everyone else just copied them. There’s nothing preventing you from using third party calendar apps if that’s what you want, and there are standard protocols to exchange calendars between services. Your email address probably comes with a calendar by default already so most people just use that, but that’s your choice.

As for IMAP, there are a few alternative protocols but desktop mail clients are old people tech anyway. Outlook is just storing your email on their servers for a reason, people don’t want an IMAP alternative, they want an app.

I don’t really know what inconsistencies email supposedly has compared to other protocols. I use a bridge to join my Signal/WhatsApp/Telegram/etcetera all in one place, and getting a consistent experience is a layer of hell not even email prepared me for. Telegram doesn’t do some emoji reactions, WhatsApp doesn’t do edits, every messenger needs stickers to be in a specific weird format, and god forbid you try to send files because every service has their own stupid quirks on that. Then there’s formatting, every service supports a specific subset of markdown, all incompatible with each other. And NONE of them allow “line of text, image, line of text” as a single message that can be forwarded. Messenger tech is bound to the same restrictions the Linux kernel mailing lists are. Email is a technical miracle in how it works consistly across platforms.

The only comparable protocols I can think of that come close to email are SMS (awful and insecure), MMS (awful and insecure and unreliable), RCS (only usable if you use Android and insecure in all other contexts). Young people don’t use email because they’ve been tricked into other apps, but it it wasn’t for the “my parents use it so it sucks” attitude that every teenager develops, email would’ve replaced so many shitty messengers.