You are right. None of us will ever be able to "clean up" our current identities on the web.
If push ever comes to shove regarding being tracked on the web, we'd have to completely abandon our current and past identities. Usernames, forums we visit, photos we've posted in the past, avatars, mannerisms of speaking, all of our online friends, mention of locations we enjoy, credit cards we've used online, etc. A total abandonment of anything we ever were. And the IP addresses we post from and the equipment we post with - our computer(s), smartphones, etc. Not even necessarily posting - simply lurking on a forum, reading but not posting, and you still show up in the webserver logs.
I could no longer use all the commas I do in posts. No more use of the word "etc" or starting a sentence with "Well, ..." I would have to change my core being - no more joking around and using sarcasm. I would have to become a libtard to hide my true political beliefs. Move into a big city. Listen to rap music. Start driving a Prius. Man, it would be pure hell to break away from my old identity.