Better bike infrastructure will encourage groups of people who don't currently bike to start. More protected lanes means more kids, more old folks, more women of all ages, will start cycling. And more cycling is a good thing!
There's many things that are good things. However, you need to balance them against the cost (money, space, time, whatever) required to achieve them. For some smaller roads, the cost might not be worth the benefit.
Encouraging people to exercise and not drive gas-powered vehicles is a net win for not only their long term health but also the environment. It's worth it.
Even if they use an electric bike and don't exercise, it's still good for pollution and congestion.
Agreed, lets skip the automobile lanes and just put bike lanes on those streets :]
For smaller streets, just make them pedestrian & bike first.
The cost of things should indeed be considered. Depending on region, tax money subsidizes every mile driven (one well done source here https://www.theatlantic.com/business/archive/2015/10/driving... but there are many, many others). A switch to cycling amounts to a much lower subsidy per mile, so if we want to talk costs then the argument for cycling infrastructure becomes even stronger.