I am sure they'd be fast enough to close the distance and counter their opponent, before he uses a spinning sidekick.
You can't go forward and retreat at the same time. If your opponent leans back he's retreating. When he leans back then you know he'll return to that vertical position and that's where you catch him.
The reason the Muay Thai fighters aren't able to do this is because they choose not to follow up. In Chinese Martial Arts, a lot of the techniques plan a followup that covers the scenario of an opponent avoiding or blocking the first strike. As out of shape as I am, I would be fast enough for someone to lean back like that and then to follow through with a spinning kick that will land.
The fighter cannot retreat or advance when he's in this lean back position. Instead of trying to land one big kick flow through it and follow up with a spinning kick. Lining back like this is actually a disadvantage. When the fighters begin to see that, they will exploit it and you'll see less of it.
Examples of the technique I'm speaking about. I personally wouldn't go for the head. I would just spin one off to the abdomen area. The time that it takes to return to the vertical position is more than en
The time that it takes to return to the vertical position is more than enough time to land the kick. For example:
A Kicks = B Lean Back
A launches spinning side kick at the same time B begins to return to vertical position.
A lands kick
The reason why this works is because B is not in the process of defending, retreating, block, He's in the process of returning to a vertical position.
Look at 0:13. You will see the move that was shown above. You also see why I wouldn't throw the kick to the head. Had he done a spinning side kick the kick would have landed. You will also see that the defender doesn't have time to retreat, the only thing he could do was to lean back again. You will see that they throw that one big kick with nothing to follow up with it but a Reset. Had the guy been better with transitioning into the spinning back kick, he would have easily kicked him in the face while he was returning to vertical position.
A lean back only puts the head farther away. Everything else is within range
There are 2 things that I know will happen without fail and without doubt. 100% all of the time unless the person fails. When a person leans back
1. They will no longer move forward or backwards.
2. They will need to return to a vertical stance.
They cannot break these 2 rules while remaining on their feet.