A friend of mine took this route in the 70's. It was not only relatively cheap but once you arrived in India you could live off the very generous UK Unemployment Benefit for a very long time as the exchange rate was incredible and the price of basics extremely cheap. He did it for several years before moving on to Japan. So he didn't need to take any time 'off' :)

Go spend some time on r/onebag. Every day there is a post by someone who is taking 3 months or 6 months to travel a few different countries. Usually younger people, but includes people who are taking a long break from work.

Or to put it another way. There were only a handful of such buses running simultaneously. So there were only a few hundred passengers per year. Is it hard to imagine that in the 3ish Billion people that live on that route today, you couldn't find 500 people who can take a few months off to do an epic journey through 20 countries?

I can't imagine spending two months on a bus.