There's a bunch of articles online on how to do it, eg:
https://www.windowslatest.com/2020/02/05/its-not-just-you-wi...
Basically have to set HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Search\BingSearchEnabled to 0
and
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Search\CortanaConsent to 0
However, I agree with you, that I would've preferd this to be turned off by default. Extra dependencies that bring no extra value are net negative, as seen in there current case where a server-side problem stopped my local search from working.
Good job, Microsoft.
I think I can agree. But the push by all browsers seems to be more search. Not less.
You can even install (and write your own) plugins for it, e.g. a calculator plugin.