I don't want you to take this the wrong way, but I want you to read what I write here and think about it closely.
I was once your age. I once cared a lot - a lot - about "honor" and "legitimacy" in video games.
I would worry about doing things "legit" by my own standards of doing things "legit" and if somebody else didn't meet those standards, then I acted like they had no right to even be claiming the rewards they were getting from their inferior, dishonorable tactics.
I assumed I was the best ever. That I knew everything, that I had the best judgement. It was extremely arrogant of me to think like that, but that's just how I functioned.
One day, a new installment in the Fire Emblem series was announced with a new mode that revived units after they had died. I was livid. How dare they take away the series' most defining element so that the game would be more approachable by more casual users? Unreal! If you can't handle a unit dying forever, then either get better at the game or just reset your map progress like the rest of us!
I was expressing my dislike of this with somebody else I knew, and he hit me with a pretty big question.
Why does it matter?
And you know what? I wasn't able to answer them.
I realized, in that moment, my very line of thinking was purely selfish. I was effectively gatekeeping the content of the game - it's characters, it's story, it's music, everything - from people that I thought simply didn't deserve to see it just because they weren't "good enough." I've stopped caring about "legitimacy" in single player games since then.
That is what this suggestion is. You want to keep people from being able to get Moon Lord equipment just because they're not "good enough" to fight the boss by your own standards of "good enough" despite, in the current state of the game, and across the many various changes to the Moon Lord to make it more difficult to defeat, the Nurse was left alone as a viable strategy for fighting the boss.
Say we put the Nurse on a cooldown. What now?
Is it a measure of skill to ask a friend to help you defeat Moon Lord with their own, post-Moon Lord equipment?
Is it a measure of skill to ask a stranger if you don't have any friends interested in Terraria?
Is it a measure of skill to have ten Heart Statues and stand in a honey pit in Beetle armor?
Besides, you talk about mods here, but if a mod author thought the Nurse was busted, wouldn't they just put a cooldown on her instead?
You're entitled to your opinions, but your opinions are rooted somewhere, I believe, is misplaced. I cannot change your mind, but I promise you that your ability to enjoy video games will be heightened if you stop worrying about how other people are playing them. After all, how somebody else is playing a game alone is not affecting your game whatsoever.
I implore you to seriously, seriously think about this for a bit. I remember that part of my life, and it was utterly miserable - if I can help even one other person get out of that funk a little faster, I'll be relieved.