Another thing about changing the tackling rules; it would benefit Gronk as a great player but it would also make tackling players like Brandon Jacobs a lot more difficult as well. Teams would find ways to get fast, powerful runners into space. They do it now but it would become a major weapon.
Do you even understand the defenseless issue we're discussing? No one that is a proponent of protecting the knees on defenseless receivers has even suggested that normal tackling rules should be changed.
You can hit Brandon Jacobs if he is running the ball anyway you want to. Head, neck, knees, dick - anywhere. The reason? Because he has a chance to either brace himself or evade the tackle or hit. He's fair game and everyone agrees he should be.
When guys don't have that ability to evade, they're defenseless, because they are either stepping into a pass or catching one. You can't physically look for the ball and find the safety coming at you from a blind spot. So the NFL gives them extra protection for a few seconds to give them time to get down or protect themselves.
Whether Gronk was still defenseless is debatable. Its not clear from the rule what the interpretation would have been had it been applied. We know he was damn close, but it is irrelevant because the current rule doesn't offer him protection for that hit.
The two issues in my opinion are:
1. Should the defenseless protection be increased to include the knee and below?
2. Was Gronk still defenseless per the current rule?
Personally, I say Yes, and Yes. But that is just my opinion.
I do think the low hits would be easier to enforce than the high ones. When guys launch at chests, the natural flow is for the offensive player to duck and helmets touch because the helmet rides up or the offensive players helmet comes down. You won't have that on low hits, you're either down there or you're not. If you're targeting the waste or midsection, you're not going to unintentionally be at knee level.