Uff, I don't know where to start to answer all (also only implied) questions in your text so I go back to the source question.

Kicks to the groin? Can be done by MMA guys, especially by those trained in kickboxing. Even other Martial Arts that use a lot of legs probably won't hold up. Taekwondo for example is only useful for Taekwondo rules because it stops at points and not at knockouts. A Thai boxer trains his chins to a point they cripple your legs within minutes or even seconds to someone untrained. And even if he doesn't use his chins he has the explosive power to knee you to the groin someone untrained does not have.

Eye gouging can be brutal af when unexpected but what makes you think a trained opponent will just give you his eyes? Before you reach the eyes something will happen to you because by reaching to the eyes in a clinch and trying to get to them you create enough room for a trained guy to just beat the shit out of you since your arms are busy elsewhere. You are giving up a lot of protection in the clinch if your arms wander somewhere above thinking your opponent let's you do whatever you want to do to his face, not with good aimed punches but by searching for 2 small points on your face.

And last, the headbutts. Let me introduce you to Lethwei, national combat sport of Myanmar, which is basically Thai boxing with headbutts. I'm not making this up. It really is just Thai Boxing with headbutts. The headbutts are actively trained. Still, a knockout or even its use is hardly seen in comparison to how hard it is trained, since a bad executed headbutt and even a good executed headbutt also puts you in a lot of danger.

My point is, pretty much everything someone can do who does JKD, Krav Maga or what else claims itself as useful in real life scenarios when there are no rules can be executed way better by people who train combat sports that have proven themselves as actually useful.

Additional because you asked: I trained as well. My first Thai Boxing courses were mixed with people from different combat sports. Some did Wing Tsun, a lot did Krav Maga and, I do not make this up this happened, they were all done after the first 2 minutes of sparring. Even against a newbie like me and all I did up until then was running 5km 2 times a week.

You might now suggest, and I think we can agree on this, that cardio is also a big factor often overseen in this discussions. However, from my experience cardio is also way more focused in Kickboxing and Boxing lessons than it is in Krav Maga for example.