I can see that you may not be happy with the proposition that the public transport system that is best, is the one that raises most revenue from those that can afford to pay.
I am certainly not happy with that, no. That is the description of a progressive taxation system, not a public transport system. If we wish to provide public transport in that manner, it should be fully funded by way of progressive taxation and should be free, or very heavily discounted (nominal charges promote respect, but I'm talking £1/through fare, £2 day ticket on all modes type levels here), at the point of use. There is an argument for that approach but I wouldn't say it is a viable option for the UK at the present time.
But I would be happy to extract an alternative criterion from your posting above; that the public transport system that is best, is the one that provides the most opportunities for regular travellers from within the city to get to where they want to, without the use of private cars. I would be happy to go with that as a general rule; would you?
Yes, that seems reasonable. I would add that that must not be to the disadvantage of those users for whom the provision of a direct service of whatever type is not viable, hence my view on multi-modal ticketing.
But if so; could you expand a bit on why you might believe that access to public transport for reluctant private car users would be greater with a single 'public transport fare'? Are there many travellers who would use a tram if the fare were the same as that of the bus; but not if it was higher than the bus?
I think so, yes, certainly in London. But the thing I find really unacceptable is the typically very high cost (until you hit the Travelcard cap) of a single journey going, in London, say, bus-Tube-train-bus-tube, or something, because of the lack of proper through ticketing.
There is a lot of survey information on travellers switching from car use to public transport use (both for Metrolink and other tram systems, and for the Leigh and Cambridgeshire busways). One interesting finding is that fare levels are not that important; much more weight is given to convenience, speed, comfort, penetration, connectivity, reliability and frequency. In so far as that may be the case, it can well be argued that there is no 'unfairness' in charging higher fares to tram users (who travel faster, more reliably, more frequently and often more comfortably, than do bus users).
I could at a push concede to a structure that was along the lines of a cheaper bus-only fare plus an all-modes fare to recognise the principle you note (and I think that may be what London would need due to Tube overcrowding). However, what such a thing does is means you need to provide for through journeys using buses otherwise you disadvantage people that way. In a proper German-style system, these are limited; buses are used to connect you to and from your nearest rapid transit railway station. And that is my model of preference.
Taking Manchester as an example, you simply wouldn't have Manchester to Oldham trunk bus routes. Oldham would have a local bus network connecting to Metrolink, on which you would complete your journey, on one through fare. And Manchester city centre would be a much nicer place without all those filthy diesel fume belching buses. Imagine a network where there was no fossil-fuel powered public transport in our city centres at all. How good would that be?
To get an idea, take a look at a public transport map for say Hamburg and see just how few routes penetrate the city centre.