MSFT was so dominant back then that Gates had to rescue Apple to keep up the appearance of a fair market!
The web killed Notes, not Microsoft.
Office 2007 introduced the ribbon and got under pressure by the web (GSuite).
But the early aughts? Office 2003 with XML and Sharepoint just wiped out all other stacks, especially Exchange steamrolling Domino and GroupWise.
I was part of a consulting team back then, the transformation projects were all going in one direction. That stack is dominant to this day.
I remember hearing that they had a storage calculator spreadsheet that would allow you to calculate how many IOPS you'd need from your storage to deploy Exchange.
Everyone I knew working with Notes or other products was gobsmacked. Why would you need that? Just buy decent storage. Nobody outside of the Exchange world cared about IOPS, unless you were dealing with of high thousands of users on one machine/cluster.
But the Exchange Server storage layer was a total dog's breakfast. (And not a healthy dog, either.) So disk performance was much more important for them.
And that spreadsheet was like crack for the senior management layer. They suddenly got to play with the numbers of users, mails per hour, mailbox quotas - then feel like they had involvement. They had input. They could go to their storage vendors with those figures and speak authoratitively about their requirements.
Those vendors would humour them, and sell them the exact same array that everyone else was buying for that capacity requirement and budget.
But Microsoft had turned a weakness into a strength, by making the managers feel like they had real input into the process.
Honestly, that spreadsheet was a genius move that it took me years to understand.