It's not about 'sharing' a bus with the great unwashed, and to suggest that this is the case is disingenuous and misses the point.
It is about a superior level of service from beginning to end. Standing in a queue with people pushing and shoving onto a bus which is usually overcrowded, and then possibly not getting a seat, does not constitute superior service. Sitting with your nose in someone else's backside and your legs at an awkward angle to avoid someone else's legs or baggage is uncomfortable and is not what premium passengers have paid for.
This is something I have to put up with when I travel EZY and other LOCOs, but if I travel premium class on a full service carrier I expect better. I suffer from (a mild form of) agoraphobia and get panicky in crowds, and this is quite common. It's a great excuse to avoid crowded shops during sales time, but seriously it can be very unpleasant and if I've paid a premium fare I will demand the right not to be squeezed onto an overcrowded bus. If that makes me a snob, elitist, grumpy old man, then so be it. End of ......