"I don't need to modify Wikipedia on the fly - I can just edit the page directly, that's kind of the point of a wiki."
Don't think the content itself. Think hostile script injection like the Chinese Cannon.
"And if I'm doing on the fly modification - I can probably make a nice certificate as well anyway"
That would take a lot more resources than just a relay. You'd need to plunk down for the certificate and to make it a dead ringer for Wikimedia would likely take state-level resources, in which case they'll just attack you directly if they REALLY want you. In which case you're staring down the barrel and are screwed anyway.
"Additionally HTTPS isn't the best solution for ensuring that data isn't modified in flight - that only requires (signed) hashes (which could be included in the page)."
But if the hashes are in the clear, THEY can be modified on the fly, too, to match, and if you have to transmit the hash encrypted, why not the whole page?
Let me put it this way. Why else were telnet and rlogin abandoned for ssh?
"Just because you have a hammer in your hand doesn't make everything a nail."
And just because not everything is a nail doesn't mean there are no nails at all. And some of them might have been nailed from the other side, leaving the room with a serious tetanus risk if you don't start nailing those points back down.