I hope they do make Cortana an "Optional extra".
I have had it turned off for a long time now, as for me, the "normal user" and not a businessman with tons of appointments and traveling a lot, just don't find it useful.

For any appointment I have, I get a toast notification from Calendar app, or can see it when I click time & date in system tray, Skype & Mail both give me a toast for communications, and you need to do a work around if you want to ask for a song stored locally on the PC to play in Groove(that's where Cortana became totally useless to me)!

In my opinion the toasts from apps are just right, with out Cortana popping up thinking you called it when you didn't, and it sometimes recording stuff it shouldn't in the background.

Oh yeah, don't forget the frustration of Cortana opening the browser search for a command that should be done, too.

Cortana is about as useless as gamebar and Xbox integration for non-gamers and Phone for users without smartphones(I sincerely hope all smartphone zombies on the streets get run over from a bus).

Oh and her jokes suck too

In the beginning she was at least useful when you could request a song and it would look on your system and play it in Groove, or open it and it would show the weather, and any news you are subbed too, but of course in Microsoft fashion, they killed the useful stuff.

Cotana beta app is now working for Windows Insiders in Fast ring and Skip Ahead.