Na issue is that with expansion distances increase. Thus you lose the benefit of being in the city (close to work, close to friends, close to cultural offerings, close to shops, close to ...) While Japan did a few things to help, like reliably working high speed railway, which allows commute over wide distance, while central places are crowded during rush hour and can't take much more.
Shinkansen (bullet train) commuting is a marginal phenomenon, with a few thousands doing it in a metropolis of 20M+. Highly reliable commuter railways with cheap express services are the backbone (and genesis) of Tokyo's sprawl.
> Thus you lose the benefit of being in the city (close to work, close to friends, close to cultural offerings, close to shops, close to ...)
This is the “Brooklyn is boring” problem. It’s temporary. The old city centre (Manhattan) is unlikely to decline rapidly in relative importance but cultural and economic life will happily extend itself from central areas to less central ones given the population and the money to make it worthwhile. Good transport links help enormously too.
Tokyo doesn't really radiate from a single center anyway. It's really an agglomeration of centers of activity. This is also true of NYC to some degree (but in a different way). "Downtown," i.e. Wall Street, isn't the cultural and social center.
