1. I don't think there's any goal of having more members rather than less.
2. If you donate, pretty much any amount at all, you'll get donor status and can delete your comics yourself. If for some reason you don't have a credit card and don't know anyone who does, or who is willing to donate on your behalf, you can probably work out some alternate form of payment with Brad.
3. Brad's a pretty nice guy. If you contacted him privately and quietly and asked him nicely to delete your comics and account, he might do it. I don't know how indulgent I'd feel towards someone who wanted me to do perform some special request he could basically accomplish by donating a few bucks, but anyway, there's nothing to lose by asking politely.
4. I am entirely in favor of people who make crappy comics removing themselves and their comics from the site, so theoretically I'm in favor of a function that allowed users to delete themselves, but again, they can do basically the same thing by donating a couple of bucks, so. One also has to consider that, as it stands, "deleted" comics aren't immediately removed from the database, partially so that if someone's account is compromised, their entire stripping career isn't wiped out. Would a user deletion system have the same safeguards? I don't know.
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The what mentioned above is total fiction. Please don't take it seriously!