In terms of the amount of time and effort required, I've no doubt "the knowledge" rivals some degree courses. Whether or not it's still useful in the days where anyone can use their phone as a sat nav, is another issue.
I think cabbies with "the knowledge" have some advantages over sat nav users. Having it all in their brain, rather than needing to refer to a phone, means they can react to requests from passengers more easily on the fly. I can imagine this is especially helpful with tourists, for example, who might want a scenic tour rather than the simple A to B that a sat nav would give.