It may be technically true that animals like dogs and humans with meat-optimized digestive systems can survive off plant material alone, but that doesn’t mean it’s advisable. Neither of us can metabolize cellulose, both of us run into metabolic diseases with excessive carbohydrate consumption (e.g. diabetes), etc.
But a vegan diet has been approved for humans at all stages of life, and the vegan dog food I'm aware of meets all AAFCO standards (nutrients, digestibility, protein:carb:fat ratio) as well as passing tests for dog interest/palatability.
There's a big difference between a well-rounded plant-based diet, and living off sugar or cellulose-heavy raw plants.
Almost every plant is nondigestible.
> But a vegan diet has been approved for humans at all stages of life
Well if it’s been “approved” then no need to worry! Who exactly “approved” it and why do I care what they say?
> There's a big difference between a well-rounded plant-based diet, and living off sugar or cellulose-heavy raw plants.
There are like 4 edible plants that don’t provide 90% of their calories in the form of sugars (starches are a kind of sugar). There are also very few plants that don’t have a huge cellulose content.