You don't just post your public key next to your comment, you use your private key to create a cryptographic signature of the contents of your post. Anyone with your public key can check that the signature is valid. Altering the text will cause that signature check to fail.

The issue here is that almost nobody actually checks the signature. Some people edit their messages to invalidate them just to see if they'll get called out on it - they almost never are.

So in theory, yes. But in practice - people are too lazy to validate or check the key (if the attacker replaces it with their own signed message) for every single post. This is a bigger issue the more users you have signing messages - as users begin getting lazier with checking each and every signed message.

Unless they are under a lot of eyeballs from people who do care. If Wikileaks "signs" a message and it doesn't verify or wasn't with their key - a lot of people will call it out. If I "sign" a message or use a different private key (very possible that I sign with the wrong key when I have multiples) - I doubt anyone would call me out on it.