City growth cannot be endless. Everything has an end. It is the same thing for Paris. With the "Grand Paris", we try to build new subways, but it is tremendously expensive.
New York stopped building infrastructure due to government and politics, not some sort of natural law.
If a city grows enough to need more infrastructure, it usually grows plenty of taxpayers to fund it.
We are also nowhere close to peak density efficiency, since the widespread refinement of high-density building would drop the price per person way below the current aggregate costs of living considering the whole of infrastructure and housing almost anywhere.
There is a strong cultural barrier to the development of higher densities though that associate it with poverty. Thats the real challenge to overcome, because that culture operates like the anti-nuclear sentiment to stymie development and stunt the growth of innovations in the domain of urban planning.