There are other things that will reduce PVR on Oxford Road besides cutting a bit of the overbussing back. The present single-door double-decker operation causes proportionally very long dwell times at stops, often several minutes at particularly busy ones like Fallowfield and the two University stops. Replace that with 3-door artics with all door boarding (or even with London style on at the front, off at the back) and it would be reduced substantially.
It's about time this rampant "we've always done this way" conservatism was challenged. For this kind of route, we need to be looking at no stop being longer than 30 seconds or so unless wheelchair access is required on that occasion - at quiet stops even shorter, ideally perhaps 10-15 seconds. That needs off-bus ticketing or a big move to contactless payment with a flat fare London-style (no paying cash to the driver), it needs wide, fast-moving doors and it needs buses with lots of space inside and no motivation not to "move down" so needs an exit door as far back as feasible. These can only sensibly be delivered using single-deck artics (or 15m single deck rigid buses if the route can be made suitable).
It genuinely is educational as I said above to compare Oxford Road with Metrobus 5 in Hamburg, they are VERY similar routes with hugely different implementation. I genuinely think some TfGM officials should take a trip over there and ride on it when deciding how to design the future of bus transport on Oxford Road.
Possibly because we are less accepting of fare evasion than the Germans? There is much less acceptance of subsidy to public transport in our culture than in Europe, and definitely no desire for taxes to increase to pay for others to evade their fares. Look at the main outcry against bendi buses in London ..... European legal and cultural practice is far different to ours (compulsory carrying of ID cards etc) which makes ticket enforcement much more difficult here.