I’ve been enlightened by this Thread as not having read the rules very thoroughly I had always assumed a valid QSO consisted of an exchange of callsigns, signal reports and finished with an exchange of “compliments”. The latter bit I picked up from something written by Joe Taylor of weak signals fame which is why an FT8 QSO includes 73s. Nothing to do with SOTA but it makes sense to me.
The exchange of 73s is a way of ensuring both sides have heard the other’s signal report. If this is not done the last person giving the signal report might then move on to another station and be unaware their signal report was not heard.
I’m not suggesting a change of the rules but perhaps 73s should be included as recommended? For example I’ve had QSOs where I’ve received an RST and sent mine but not heard anything more. Was that a QSO?