Windows 10 just updated a few days ago and my Opera was utterly erased from my computer.

Do you remember where operamail.exe was installed at? It should have been at "C:\Program Files (x86)\Opera Mail\operamail.exe" (or "C:\Users\yourusername\AppData\Local\Opera Mail" if you didn't tell the installer the correct spot) where the roaming and local profile folders for it were at "C:\Users\yourusername\AppData\Roaming\Opera Mail\Opera Mail" and "C:\Users\yourusername\AppData\Local\Opera Mail\Opera Mail", where the mail folder was specifically at "C:\Users\yourusername\AppData\Local\Opera Mail\Opera Mail\Mail".

If you installed Opera Mail elsewhere, like in "C:\Program Files (x86)\Zip Zam Bam", your mail folder should have been at "C:\Users\yourusername\AppData\Local\Opera Mail\Zip Zam Bam\mail".

So, that should give you some hints on where to look. Of course, as already said, you can just use windows search to search for all mail folders. Just search for accounts.ini or omailbase.dat. Or, search for all mbs files with the *.mbs term. You can also search for operamail.exe to see how many you find.

If Opera was installed in a spot that gets overwritten during updates/upgrades, that could explain it.

Under "About Opera Mail" it says, "Version 1.0, Build 1044, Platform Win32, System Windows 8"

Ah, Opera Mail.

At that same page you should see a path to 'mail'folder. Your email messages are stored there in the 'store' (or 'storage') sub-folder.

I reinstalled it, but what's to keep Windows from doing that again?

It may not be easy to say without knowing exactly what happened.