Re: And integration...
"I can certainly make an argument that the day to day TCO of Exchange Server is lower than Lotus Notes"
Maybe - but only in the context of an email platform. Messaging is only one of the services it provides - at a high level, it's also is an authentication server, application server and web server. Yes I know you can probably get the equivalents for free if needed, but they will never work as well together as Notes/Domino, with everything under the same hood, and all connected by the same coding frameworks.
Furthermore, when you get into the details, if you were to replicate the functionality of a Notes/Domino setup with MS equivalents, you'd be looking at AD, Exchange, Sharepoint, Teams, Power Apps, Power Automate, IIS... the list goes on. Not sure if the TCO of that lot would be lower than Notes/Domino!
In fact these days, the messaging component is rarely used now - Domino's mainly used is as an application/web server.