I have news. I just opened a chat session with Microsoft Support. They were actually nice enough to give me support for this free Windows 10 upgrade.
What we discovered was that, while none of the mailto links from ANY browser would work with Pegasus, if I switched to Thunderbird as my default email program in Windows 10, THEN the mailto links work!
So this positively identifies Pegasus as not working with mailto links from all browers, on my PC, after the upgrade from Win 7 to Windows 10.
It MAY be because that WSEND.EXE is a 2011 dated file?
And yes, I've ALSO tried running WSEND.EXE in Win 7 compatibility mode, to no avail.