That would seem then to be the most obvious convenient fix- make it possible to ban people without deleting comments. Or even better, have a button which bans them and deletes all comments that don't have replies. That way the only comments removed are those which didn't have any discussion associated with them.
A brief line from this comment indicates that the author of the cryonics-critical comment quoted here was perhaps not the one that deleted it.
Was it deleted by a moderator?
Honestly, the decisive downvoting seemed to do the trick of hiding it from casual readers who don't want to see the long annoying rants. I don't think it was casting any doubt on the credibility of cryonics.
While it sounds like the author regrets posting it, I would think they should be allowed to delete it themselves.
Edit: Originally titled "Cryonics critical comment deleted?"