Replace the spindles with what? Seems it would be harder to find French-threaded spindles for clipless pedals than to find the French-threaded pedals(?).

I went to a lot of trouble to source clipless pedals with 1/2" threads for several Schwinns and a Huffy. They haven't been made since the 90's.

If one taps a French-threaded crank for 9/16" pedal threading, how good is the resulting threaded hole for using a French-threaded pedal?

I've been meaning to tap a junked French crankarm to 9/16" and than test the threads with a French-threaded pedal and a big pedal wrench.

If the crank is steel, I'd be quite surprised if a French-threaded pedal couldn't still be reliably used, but I am very curious as to how tight and securely that the attachment would be in a re-tapped alloy crankarm.