It occurs to me, when you mention that selling a T-shirt is a lot like publishing a book, that the publishing model has actually been invading the "monetize your webcomic by selling T-shirts" business. The main example, Topatoco, has over 30 very popular webcomics creators in its stable. It's also the case that T-shirts are a standard part of webcomics culture at this point, which is not the case with books, except perhaps for books that have become blockbuster movies.