Think about integrating calendars, corporate contacts (from AD), handling RSVP replies said mx server receives and updating the calendar server, securely deal with modern auth (+ legacy krb5 auth, yuk). It's a huge hassle and everything except Exchange only handles 80% of this.
Modern expectations now want: web clients (OWA), todo lists, integrated storage (SP/OneDrive), and push notifications to any phone from any vendor.
So yeah, the only on prem solution is still Exchange.
RSVP for example. Nobody read or cares who and what people reply. In the last 4 companies I worked for (including one in Switzerland), nobody cared if I accepted or confirmed my attendance to the meeting and would try to call me/force me into a meeting even when my status showed I was on another shsring my screen. And nobody seems to respond nowadays nor check calendars for availability and avoiding conflicts.
to be honest, most things you list can be setup with some research. The only one I am not sure about is integrated storage, but then I am also not entirely sure what that even is supposed to mean exactly
So, passwords are bad and the password is a key component of krb. Moving away from passwords is a step in the right direction eg OIDC.
i give you the mobile part, I dont know how well it is supported - iOS claims to have support though, and android through third parties I believe. Never tried that. Its just that I personally have a preference for auth methods that dont require opening a browser for desktop apps