I don't think this replaces email in the way PG intends. In a way it's the opposite, this is a way of saying "I've got your number so I am going to force my way

I don't think this replaces email in the way PG intends. In a way it's the opposite, this is a way of saying "I've got your number so I am going to force my way to priority 1 in your queue over all the other email/messages you have". I think that is fine if you are wife/husband/mother/daughter/best friend etc. but not something I'd expect a coworker to do to me. Maybe a boss (a bit like a manual pagerduty scenario).