It is a pretty simple matter to make a supplementary post if any edit is required.

There was also concern that people could renege on deals or change terms and conditions.

Yes they can delete if they used a password, although I have mulled over the idea of restricting the time-frame in which they can do that (maybe 24 hours) as most deletions appear to be where the poster made an error, rather than changed their mind.

I have on a rare occasion 'locked' a post that contained something that may give rise to a complaint so the poster could not remove it. That is something Greg also did in the past.

It has been thoroughly thought through (sounds like a tongue twister) and there are many reasons I prefer to leave it as it is with the edit function switched off.
If something really does need valid editing, the poster can always ask me to do it, and this does happen occasionally (although most people would never know).