But... they work. They're not unsustainable. If they were unsustainable, the very example you cite wouldn't exist. Hell, even really small websites make a killing purely by soliciting donations. Maria Popova of brainpickings does it and makes an unbelievable amount from it (though, she does it by misleading her users and doing ethically dubious things here and there). There's a balance to be reached here, but I don't think it's particularly difficult to pull off.
> I have an intense distaste in the idea of fragmenting the community into "those who pay," and "those who don't" (a la Reddit Gold)
That depends on the implementation. Reddit did an awesome job of rolling out Reddit Gold, it most definitely did not fragment the community.
> Which leads us to what probably the main reason 4chan is a shitty business: because I choose for it to be one. If I'd decided to rape 4chan for all it was worth, as many would have
I don't know moot. Who's to say 4chan would've gotten as big if you were more involved in getting money out of it? Seems to me, that considering the userbase of 4chan, another place like 4chan but without the ads would've been the go to place for them if 4chan wasn't very much like 4chan as it's been. User-experience matters in getting the users -- that's why Reddit won, that's why Imgur won, etc. etc.