>It's out of the question in most places, it slows car traffic down
If you actually make a sufficiently high-quality bike network to convince people to bike that would otherwise drive, I would expect this to speed car traffic up by taking a lot of cars off the road.
NO
I like the comfort of my car too much to freeze my ass on a bike. Cities were built around the vision of economic prosperity and a car for everyone, and they should stay that way.
This is very geography/climate dependent though. There are conditions where most people would not bike regardless of how high quality the bike network is.
Not everywhere is San Francisco. Where I live, it's below freezing almost half the year. No one wants to add bike lanes that will only get lightly used 3 months a year.