100g of wheat gluten is almost 80g of protein. Which easily provides the amino-acids most legumes have in lower concentrations. Besides seitan which is very easy to make from wheat gluten, tofu, tempeh and many other plant based (and really easy and cheap to prepare on your own, in large quantities and store in the freezer, I have a drawer in mine full of them) foods in some traditions that are even low in carb content for people with insulin resistance. Or just low in calories so you can cook them whichever way you want with whatever else you want to add to your food. It's not really hard to actually do it.

Being "not flabby" has more to do with your understanding of how insulin works on the body than it has with anything else, even activity levels. Especially in western societies. And its pretty damn hard to think properly about your food, even though its scientifically clear those in charge are pushing shitty guidelines to the people. For example, as you can see here :

Conflicts of Interest: First-ever systematic review of the extensive financial conflicts of interest on the Dietary Guidelines Advisory Committee. Findings: 95% of the 2020 committee had at least one tie to a food or pharmaceutical company; Over half had 30 such ties or more; USDA does not disclose conflicts of interest, despite a National Academies recommendation to do so.