Here, robust transit has solved the last mile problem for most people.
https://cwhong.carto.com/viz/6dfca01c-47e5-11e6-9fd3-0ee66e2...
Weasel words are doing a lot of heavy lifting here. There is a persistent last mile issue even in NYC, even in Manhattan. You're right that in Manhattan most people can use the subway as a last mile solution. However that map hasn't changed much in quite a while. The subway deserts that exist (in Manhattan and the other boroughs) aren't going away anytime soon because building new subways is eyewateringly expensive.
The inflexibility means that even when the subway is a viable last mile solution it may not be the appropriate one. For instance I had to go from Ridgewood to JFK a few years back. I was maybe a five minute walk from the subway. But were I to take the subway from one end of Queens to the other I would've had to go all the way to Midtown and transfer to LIRR.
Hell I've generally had to rely on buses for last mile connectivity even in London which certainly doesn't suffer from a lack of subway service.
This has not been my experience since moving to Manhattan last January. Subways, alone, close the gap between regional rail and most destinations astoundingly well. I haven't yet needed to use a bus (but they seem abundant, too), and I haven't even thought of taking a taxi yet.
Here, robust transit has solved the last mile problem for most people.