"I consider this a problem. It may be a fun power fantasy, but the game clearly loses most of its challenge, when it's supposed to be at its most challenging at this point. I'm open to more suggestions regarding overpowered aspects and ways to correct them."
When you have 60k hp in legendary difficulty, and monsters hit a max of 2k, there is no difficulty. It's just easy. The gameplay is watered down to clearspeed only. So, I agree with you. And, the current state of the game is much worse than what I have in that video.
My recommendation is to make the higher difficulties substantially harder. However, difficulty can't be blindly raised. A serious, comprehensive balance overhaul would be necessary to make various builds viable.
In Zombasite, my 'mo mod' was meant to address this. It was a crap-ton of work that I would not easily repeat given my current circumstances. If such balance was applied to the game from the Developer's Perspective, I could see the game having even more replayability due to more build diversity while simultaneously being genuinely challenging.
But, I can understand why Shadow wouldn't want to do such a massive overhaul. It takes a lot of time. And, he might think the game balance is fine enough as is, given the amount of players that the game has. This is why I made my post in the feedback thread for a veteran's mode that has much accelerated scaling, for the people that want a real challenge without being forced to mod the game, and without a huge amount of work being necessary to make difficult gameplay.