SYG only drops the requirement to retreat before using lethal force in self defense. Retreat-first laws required people to put their lives in danger; even just turning to run is more than enough opportunity for an attacker. People should never be criminals for not letting themselves be victims.
If people were using deadly force without being in danger, that is a fault of the self-defense laws, not SYG. In most of the cases you mentioned, the main issue you seem to have was that the attacker was retreating. It's the difference between danger and immediate danger. In immediate danger, retreating would increase the chances of bodily harm (why we need SYG). In a vague sense of "danger" without an immediate threat, you have no claim to self-defense or SYG; you're supposed to retreat in that situation, even with SYG on the books. In these cases, it seems that they were successful in claiming that they were still in immediate danger. Whether that is true or not, we'll never know. I think the legal system fails us on a lot of these cases, but it's the prosecutor's that are failing at applying the law, not the law itself. The flaws in SYG are small in comparison to the flaws in Retreat-first.
If you could show me evidence that SYG is overwhelmingly being used by criminals whilst in their art vs. innocents trying to stay alive, I'll concede.