Yes TfL has started letting you bus it even if you have only one penny's worth of credit. And sure, if you're lucky enough to have a contactless debit card, you're probably fine. But let's face it: at some point, most of us are going to want to get a night bus, having run into negative Oyster balance. And then what, eh?
Public transport is an essential part of the infrastructure of any civilised society and therefore if disabled people are to achieve full equality as contributing citizens, it is important that public transport is as accessible as it can be, and by this I do not just mean wheelchair access but also a whole range of features so that transport is accessible to a wide range of people with differing impairments.