Deleted accounts could retain the comments but anonymise the accounts.

Also I don't think having the ability to delete your profile invites trolls. Usually one deletes their account because they post something wrong but which they genuinely believe is true then get downvoted and / or trolled into oblivion. These people we should probably be working to retain because while they might have been wrong, their contribution did improve the content on the site due to the corrections that followed. However sometimes the negative rep and / or harshness of the replies can make the corrections a bitter pill to swallow. So I think there should be a downvote cap for incorrect posts (or even disallowing downvoting for all but rudeness and spam) and working harder on improving forum etiquette.