I'm not vegan, but I think that going vegan just to eat seitan sounds really awful. There are plenty of foods that are completely vegan that would make a great main entree. Obviously, you're going to have to eat processed stuff if you are trying to imitate something that is not actually part of the set of foods you are willing to eat. That's like if I wanted to give up potatoes for beef but still eat tater tots -- I'm going to have settle for something heavily processed.

On the other hand, if you just ate food that was naturally vegan, like lentils and rice. You would do just fine, and it is actually much easier to cook than chicken, since you don't need to worry about contamination, etc.