1. All of the software that creates printing paths is a derivative of the work done to turn solid models into milling machine tool paths. That software has existed for well over 30 years, and the proprietary stuff is excellent.
2. This is the big advantage of 3D printing. Arbitrarily complex shapes don't increase the cost. That said, you can do some insane things with a 5-11 axis machining center.
3. It's impossible to say as a general rule which one takes longer. If you have a shape that is relatively close to the full volume of the stock, then mills will be faster than 3D printing. If you are spending a lot of time cutting down stock to get close to the final shape, then 3D printing might be faster. Either way I would expect finishing steps.
4. Either way you are starting from scratch, this goes back to #3. No one really wants to do rework unless you are building very expensive industrial parts in low quantities, and at that scale you are unquestionably milling.
5. CNC mills are exactly the same. Actually most of the ones in a factory take the raw feedstock in and spit out finished parts, no human intervention required. 3D printers generally don't do this yet.
I get the impression you haven't worked with advanced CNC machining centers. I've worked with $50,000 mills (and $500,000 mills). I've also worked with $50,000 3D printers. The $50,000 mill beats the pants off the $50,000 3D printer every day of the week.
At the hobbyist level and a budget of $2,000 things are different, but serious industrial production is definitely better done on CNC milling machines. If you are comparing manual mills to CNC 3D printers, that is pretty apples to oranges. That would be like comparing a CNC mill to a guy holding a hot glue gun.
Source: I've been a mechanical engineer designing manufacturing systems for medical devices & nuclear submarines.
Source: I have background in these field as well.
1. You have to manually do all the work, or at least set up tool paths which is a pain (and good software for this is even rarer than good solid modelling software).
2. You can't do as complicated shapes. I'm not talking fractals, I mean can you mill a plastic latch as easily as I can 3D print one? No way.
3. It just takes longer. Once I've done my CAD with 3D printing the work is basically done. With machining it's only just beginning.
4. If I make a mistake in my model and have to make a change, it is extremely quick to just do the modifications and reprint it.
5. If I want more than one of them I just click print again.
You're right that good 3D printers are still quite expensive (FDM ones aren't really worth it, but there are SLA printers for a few grand now). However good milling machines cost a similar amount, and CNC milling machines cost more.
The image problem with 3D printers is because all the cool and actually useful uses are one-off and in companies so they don't get talked about as much trinkets and jewellery. For example I made a mount at work that attaches a stepper motor to a linear slide. Can't release it to the world though.