At least buses in Britain have plenty of seat provision, which is not what I have experienced when in Greater Manchester when using the Metrolink system. Perhaps can someone say whether the original T68 trams there had more seating capacity than the M5000 ones currently running there.
The T68s had 82 seats + 4 fold-down seats in the wheelchair bays. The M5000s (from 3075 upwards) have 60 seats plus 6 'perch seats". Both tram units have a nominal total capacity of just over 200 at 4 standing per sq m; but the M5000s (from personal experience) are a lot more comfortable to stand in, and at a push can run much fuller at 'crush' densities.
Standing in buses is very different from standing on trams; so long as passengers have plentiful handholds provided (the latest M5000s are a forest of grab poles) many passengers actively prefer to stand in crowded trams at speeds of up to 80 kmh. If a bus (like a tram)is to carry most of its passengers standing at peak periods, then it cannot realistically travel faster than 45 kmh.
But that is not a serious restriction for a future metropolitan bus network; as most inner city roads are likely to have 20 mph speed limits anyway. So long as those 20 mph limits do not involve intrusive speed bumps. But for longer BRT-style trips, passengers will have to be seated. However, for shorter distance travel, the trams demonstrate that most passengers will happily pay full fare to travel standing up.
But you have raised a good point; overall bus networks in Greater Manchester - even at peak - run seriously under-capacity. Standard double deckers have a capacity of around 90 (typically 78 seating, 15 standing); yet average peak occupancy on most routes is between 25 and 30. The exception, of course, being the busway; whose buses have 63 seats and 28 standing places; and for which all seating is full on peak serivices.
Which , in my view, shows the way forward. In the deregulated market, buses have too many seats, rammed too close together, which are very rarely fully occupied. They have insufficient space for standing, buggies, and wheelchairs. I would expect GM to shift to using two different models for stopping bus services; on the very high volume routes (42 and 192) I would expect vehicles with many fewer seats and a lot more standing - maybe single-deck and articulated - with multiple doors. On longer distance lower volume routes, I would expect something more like the current busway units to predominate. But both models would be low (or zero) emissions.