The problem with nuts is that they simply don't grow everywhere. Up here, even hazels are small and scant most years. Though chestnut tree grow well they don't produce viable fruit. Beech even some years is full of empty husks. Late frosts, high winds etc mean that protein of any sort is hard to grow on plants. Broad beans are a good source, but they're not a year round supply and if they were stored we would have to dig a lot more ground and go the monoculture approach to provide for all those parts of the U.K. That aren't the south. Eggs are a good source of protein, so is cheese, and meat. You won't grow a thing up here if you don't think about the rabbits first. For me it's a no brainier- I can eat the rabbit and save enough of my veg to eat. Although rabbit gets tedious after a while.
Yes, it's a good idea to think through all the nutrition we need for every local climate. Thereotiecally in the uk it's possible to grow and live on veg, but not so in so many other parts of the world. But, if all arable land in the uk was given over to cultivation we'd be emvironmentally poorer for it. And we would lose a lot of nice bovines, pigs etc. There is simply no room in a society built on the bottom line for mega beasts that don't serve a purpose (look at shire horses as an example). The veg argument tends to gloss over potential loss of some beautiful creatures for ever, which is a problem that in my mind needs a solution too!
Permaculture solutions to just veg require green manures (same as for livestock except just turned into the ground. The two methods are so closely intertwined I don't think it makes sense to extricate one and try to get it to stand on its own. We have good and ethical solutions but it requires understanding that like it or not our lives are linked not just to the soil but to other animals, we are apex predators and do require the sustenance that though veg can provide, meat provides better (as a part rather than the whole).