Looking at it the other way . . .
"At this point in your life, Outlook may be familiar, which we can often confuse with productive or well designed."
On the flip side:
"At this point in our development cycle, Y!Mail is new and shiny, which we often confuse with productive or well designed (and, importantly, an improvement over existing solutions)."
The thing is that whenever a product is redesigned or replaced, the teams developing it and marketing it take the unjustifiably arrogant and offensively condescending position that anyone who doesn't love their new software is unreasonably adverse to change or living in the past.
In this instance, Yahoo! continue to hold this view despite the fact that not only have their public users rejected their new mail, so have their internal staff. So very telling that they resort to taking shots at the software being favoured over their own solution, as well as MS itself and imply that it's staying power has everything to do with users being stubborn or lazy and nothing to do with fit and functionality.
That said, I do applaud Mr. Roumillat's memo - very well written and largely geared towards convincing users why they should adopt Y!Mail rather than why they should abandon Outlook (previous comments aside). Still, I would have loved to see him write: "If you are resisting change, please let us know why - If there are any tasks you feel are easier in Outlook than Y!mail, or important functionality missing, we'd like to know".
Finally, while I don't know how they got their numbers, if it was a user survey or a usage chart then it's possible that many of that 75% did give it a shot and decided to go back.