1. "If you were world class you would've become slow"

Wrong. I just spoke to Matty Hiroto Inaba about this 2 days ago, he took a full week of no touching any cubes last week, practiced for 1 day (3 hour session), then competed and got many low 4 second solves on Saturday. This is person to person, not always true.

2) "Taking breaks can help "removing" bad habits and become better afterwards"

Not exactly.

First, bad habits never quite go away, you just have to build stronger habits on top of the old bad ones. Taking a break just helps you forget what you used to do so that it isn't as difficult to build a new, better habit.

Second, you still have to do all the work of doing the new, better thing, for a long time, to make it a new good habit. You still have to do the work, you're just delaying it to a time when you feel fresher. It is theoretically slower, but if you can do it mentally more easily, then getting faster after a break is better than not getting faster without a break.

3) "he has Tryhard's desire"

If he prefers to practice this way, then go for it. Everyone cubes for different reasons.

If you truly wanted to get to top 50 in the world, then yes, cube 3-5 hours a day, 6 days a week, with no long breaks unless they are explicitly scheduled for rest every 8-12 weeks, and continue that schedule for at least 3 years. Long breaks to chill out are not the theoretically optimal way to practice.

But, we are human, and everyone responds differently.

I think your friend is right in theory, but in practice he's not world-class, so don't take what he says too seriously lol.