If something looks like it's an impossibly good investment, it may actually be.
It's a shame that US city governments let this happen by retaining the medallion monopoly system. If cities let the taxi supply adjust to demand, a taxi license would have been worth the same as any other business license.
If a taxi driver quits, the city, and not the previous driver should choose who is the best candidate to replace them.
This is essentially the same problem that we see reoccurring everywhere in modern society. We put in a system that seemed like a good idea, but after some time being exercised in reality, turns out to have some serious flaws.
Fixing those flaws is so time consuming and expensive that it becomes a bureaucratic impossibility. The result is that the profits pass onto the few, and everyone else suffers for it. (e.g. politician buying in Washington, software patents, prop 13 in California, etc.)
Probably not a fair criticism of the investor in this case. 20 years ago, it would have been a totally accurate assessment.
I'm not saying there are no limits but land shortage is only an issue in city cores.