(defun modi/region-extract-function--C-u-kill (orig delete)
"When a region is selected,
and if \\[universal-argument] is used,
and DELETE is \\='delete (when doing \\[kill-region]), or
DELETE is nil (when doing \\[kill-ring-save]),
kill the region with all trailing whitespace removed and also replace 2
or more spaces with single spaces.
Else, execute ORIG function."
(if (and (region-beginning)
(eq 4 (prefix-numeric-value current-prefix-arg)) ; when using C-u, and
(or (eq delete 'delete) ; when cutting (C-w), or
(eq delete nil))) ; when copying (M-w)
(let ((sel (filter-buffer-substring (region-beginning) (region-end) delete)))
(with-temp-buffer
(insert sel)
(delete-trailing-whitespace)
(goto-char (point-min))
(while (re-search-forward "\\s-\\{2,\\}" nil :noerror)
(replace-match " "))
(buffer-string)))
(funcall orig delete)))
(add-function :around region-extract-function #'modi/region-extract-function--C-u-kill)
So now it is probably more immune to future changes to the default value of region-extract-function. Thanks for the tip to look into add-function.