| I'm guessing the "Windows 10" is to compete with "MacOSX".
| And that means all future OSes will be 10.1, 10.2, 10.3, etc.
| Maybe it's acknowledgment of a "subscription future".

All of that seems like a good guess. Also, Microsofties
have always been oddly infatuated with "X": ActiveX,
X-Box, docx. The Win32 API has many functions that
have been updated, which MS denotes by "ex". For
example, GetVersion returns the version of the OS.
GetVersionEx is the newer, improved replacement. It's
used in a similar way as the word "premium", having no
actual meaning but implying some sort of specialness.
Perhaps even the ditzy MS obsession with XML goes
back to X.

With Windows being at #10 they can plaster Xs here,
there and everywhere, "to their hearts' content". Who
wouldn't want to replace Notepad, with Notepad X,
after all? It sounds like a race car. Notepad X is sure to
be more spiffy, with great new functions like a built-in
Bing Bar that searches thesaurus.bing.com. The Bing Bar
could then send a copy of your work to Microsoft every
few minutes to help with picking ads to show at the top
of the Notepad X window. Then the cover story can be
that Notepad X is cloudified (or rather XCloudiffified),
with online backup. I can hardly wait. Progress is a
thrilling thing. I'm already feeling a bit depressed that
I'm just writing this post in plain old OE and not in
OutlookExpressX.