Scaling things almost always makes things more efficient. Cooling a data center that does the work of a million desktop is going to use less energy to cool it than those million desktops.
But, peertube won't ever be a replacement. Most youtube creators do what they do for money. Peertube isn't the platform for that, and I'm not convinced the storage and hosting requirements will really ever be a proper replacement for youtube. What youtube does is much more expensive than what reddit or twitter do.