It's why you're not supposed to be able to tackle from behind. Now the way the rules have been interpreted for a long time is that you're right, it isn't a foul normally, but it is when someone gets serious injured. Is that right? Probably not. But the way to make it right is probably not to ignore tackles that break players legs so much as it is to penalize it when it doesn't break players legs and I don't really think people are going to like that.
Even an incident like Son's which no one is arguing isn't a foul is usually just a yellow at most. But it is still in inherently dangerous tackle that is largely just ignored as an common foul or maybe a yellow except when it goes disastrously. I don't know what the right answer is. Stopping those challenges seems like it might be a good idea but the game changes and people can't handle that. So I think you just have to leave it and hope players don't get hurt seriously.