While, averaged out over time, it may in theory meet the need, in practice there will inevitably be times when a queue builds up, especially at closing time. Many years ago at school, I studied Queuing Theory in maths. I don’t recall much about it, but one thing I do remember is that reducing the number of nodes may not impact too much on the average wait, but it can have a much more significant effect on the maximum.
Plus, without going into too much detail, not every visit to the loo is over and done with in a couple of minutes. Sometimes it can take ten minutes or more. If that happens, it may place those left outside waiting in an excruciatingly embarrassing position. When you gotta go, you gotta go, and you can’t hold it in indefinitely.
It can’t be denied that the rise of micropubs has injected an element of freshness and enterprise into the pub scene, and I certainly wouldn’t want to see minimum toilet provision for pubs and bars laid down in law. But, at a personal level, I really wouldn’t choose to spend much time at all in any establishment with just a single WC. Even the smallest traditional pubs, such as the Circus Tavern in Manchester City Centre, manage to have separate gents and ladies facilities, and to offer a choice of urinals or trap in the gents.