Skin elasticity is directly useful. Stronger bones are probably less likely to break, although this isn’t quite a given. Improved mitochondria are improved mitochondria and are not at all guaranteed to be beneficial.
Improving ATP efficiency is huge... i agree with you it may not mean stronger bones and more elastic skin...but we do know aging (less elastic skin and weaker bones) is related to the bodies breakdown in ATP supply/demand/conversion.
I just don’t think you can gloss over mitochondria inprovement as no benefit...it’s literally an improvement in energy (more energy with less calories), it’s a benefit unto itself.
In cancer, we see all the time the ability to get a cell we know is important into a tumor does not, necessarily, associate with clinical benefit. The same goes here. The ability to improve one small piece of biology does not mean, necessarily, that you will see a larger, emergent effect. Biology is much more complicated than that.
No...there are numerous studies on the proven benefits of improved mitochondrial function/capacity. One of the benefits is cellular anti-aging.
Also, you’re assuming that “cellular anti-aging” is good for your health. You could be right, but you could be wrong.
It's entirely plausible that, in the modern world, this is actually a negative.
Also, for all anyone knows, just "improving" mitochondria might also hurt due to increased production of free radicals and other strongly oxidizing species by those improved mitochondria.