Huh? It's fairly obvious that the pedals on the OP's bike were never properly tightened at some point, so any stress that caused thread damage was from force being applied was because it was loose, and it eventually wore.

I've always had a problem with the thread orientation of pedals. Not a problem on the bench, but why they chose to reverse the threads on one side or the other, Think about it. If the pedals on either/both crank arms were the same, there would be no more failare rates than what we see currently.

The orientation on pedal spindle threads are designed to tighten the spindle to the crank arm if the bearing fails, and it will, but why? I'd hope that one would recognize a bad pedal bearing long before it was "tight enough" to unscrew the spindle from the crank arm.

There is absolutely no reason to reverse the pedal spindle threads on one side of the crank vs. the other. When you install pedals you more likely use a wrench with a lever of about a foot long. Hopefully you've torqued it properly. Assuming the pedal bearing freezes, the lever in place that might "un-loosen" it might be what- 1 1/2- 2"?

That being said- The damage to the crank arms shown in the OP's fliks ae repairable with ether a heli-coil or a product similar to a Keen-sert. It is nowhere beyond the damage that either method would not affect a "like new" repair. Can't get it done at your LBS? Go to a shop where they know what's happening. Cost will most likely be in the $20/ each side neighborhood.