I have never seen any sort of positive facial shortening as a result of extended tooth contact. Shortening, yes. The good kind? Debatable. I have had my teeth in contact all my life and ended up with a Class 3 Maloclussion regardless. I know others can attest to this as well. As if mouth breathing was the only reason people have bad facial development.

why wouldnt chewing something legit hard like mastic everyday with all teeth be better than clenching

Clenching has a lot of negative health effects, such as receding gums and TMJ problems. 

Besides, it doesn't lead to the kind of shorter face you want. It shortens your molars (they call this a low bite) and you lose bone in the alveolar bone and maxilla (If I understand correctly a little bit of pressure preserves/creates bone, while a lot of it destroys bone). So your face will collapse vertically.

See here how a face changes when bone is lost:

https://www.deardoctor.com/articles/hidden-consequences-of-losing-teeth/

See here how a longer face can sometimes be more beautiful:
https://www.faceliftdentistry.com/before-after-photos.html

Posted by: @jelly

Clenching has a lot of negative health effects, such as receding gums and TMJ problems. 

Besides, it doesn't lead to the kind of shorter face you want. It shortens your molars (they call this a low bite) and you lose bone in the alveolar bone and maxilla (If I understand correctly a little bit of pressure preserves/creates bone, while a lot of it destroys bone). So your face will collapse vertically.

See here how a face changes when bone is lost:

https://www.deardoctor.com/articles/hidden-consequences-of-losing-teeth/

See here how a longer face can sometimes be more beautiful:
https://www.faceliftdentistry.com/before-after-photos.html

Only thing I have issue is the whole "the lower third is smaller, creating an aged look". Looking like you have aged isn't tied to lower third height. What it is tied to is soft tissue support, which this person has entirely lost. It's also the monkey mouth created by the long philtrum. 

Actually a short lower third is present in children. So the proportion itself isn't the problem. Women can have short lower thirds their whole lives and look cute if their soft tissue support remains. 

1. My "take away"-point wasnt, that we all should practice clenching (as I am aware of the possible negative side effects, which I mentioned in my post), but rather to explore the possible relationship between the duration of the contact of your teeth with lack of forward-growth and too much forward growth. 
If I remember correctly, the question if the teeth should be in contact while mewing was an ongoing debate not long ago and maybe the little detail of light contact vs no contact at all can make a lot of difference.

2. "why wouldnt chewing something legit hard like mastic everyday with all teeth be better than clenching"
The simple answer is: duration. Chewing for 8 hours a day is harder than having a slight contact between your teeth for 8 hours.

3. It would be interesting if intermittent intense biting (50-80% of max) on something soft (towel for example),
could jump start mewing by "losening" the sutures and/or if the teeth or the lack of them sends signals to the bone to deteriorate, maybe pressure on the teeth also "send" signals to grow the bone. 
Its of course only guessing and hypthesizing, but so is thumbpulling, facepulling, bagpulling etc.

Edit: Intense chintucking forces your teeth to be in contact. 

Posted by: @fiver

why wouldnt chewing something legit hard like mastic everyday with all teeth be better than clenching"

My theory:

Because almost no one has perfect occlusion. So clenching teeth together creates uneven pressure along the bite worsening it.

Chewing hard gums may even out those imperfection distributing pressure better

Actually this is my reinterpretation of what an implantologist said to me concerning receding gums. Malocclusion generates micro traumas that could wear out tissues

So my advice could be: the worser your occlusion is, the less you should keep the teeth in contact. 

@kyte

I agree, I think it's bad advice to recommend people to have teeth in contact when so many people have messed up bites, what is it, like 95% of people in westernized countries have some sort of malocclusion? You also have to take into account teeth crowding especially from the front teeth, trying to meet your teeth if they are crowded isn't going to work, and for the most part if you have a malocclusion you have crowded teeth.