Globaliser, I don't know if you saw the news this week that the over 60s now outnumber the under 16s in the UK. The relevance of this fact to the debate on this thread is that there will be an increasing number of 'less mobile passengers' in the future.
As for lifts on the Tube, I don't know if you have heard of Metcalfe's Law of networks. Broadly stated it says that the usefulness of a network is proportional to the square of the number of nodes. As as been pointed out elsewhere the current provision of lifts means that there are very few useful journeys that can be made today on the tube. But that is not a reason not to start the process of upgrading the network. BR managed it quite successfully, having started 40+ years ago.
My father has been in a wheelchair for 25 years now, and has suffered the indignities of being strapped to a back board so that he could get off a flight at Faro. He gave up on flying as it was just too much like hard work.
Coming back to your point, it is a very good question what is the best way to provide transport for wheelchair users in a busy city like London. Taxis don't work - yuo can't just assume that there will be one on the street when you need it, and have you tried pre-booking a wheelchair accessable taxi? If you can book on, it often won't turn up on time as they are so few and far between. Buses are getting there, but only if they bother to get close to the kerb. Private cars are really the only option, except some ***hole has parked in the disabled spot.
Simon.