My son & I got into the hobby about 4 years ago and we love bringing broken machines back to life. Anyone who thinks fixing up machines and selling them at a profit is a good way to make money is sadly mistaken. If you consider purchase price, parts, time, etc there is no real money to be made fixing up machines. You could make more sweeping floors or flipping burgers than you can bringing these machines back to life.
As far as new in box machines - lately there has been a trend in most of them going up in value but that has not always been the case. Look back at some machines that have tanked. Get it right and you could make a bunch of money but get it wrong and you could take a huge hit. Does it make sense to take that chance if you have extra cash around - everyone has to make that call for themselves but again logic says it is risky.
As far as the "flippers" making money. I buy most of my machines from a local flipper. I would prefer paying him to deal with driving for a machine then having to drag it out of someones basement while dealing with their dog & kids in the way. The first year I was in the hobby I wasted a lot of fuel and time running for machines and once I got to the sellers house machines where not as nice as they said they were - etc. Now I just head to his place & pay a little more to do things the easy way.
Pinball machines as an investment or a business just don't make a lot of sense no matter how you look at it. But it is a great hobby and with prices going up each year it is one of the few hobbies you can be involved in that doesn't cost you a lot of money when you are ready to move on to something else.
Yes - there are people in it just for the money but my experience and business knowledge tells me it isn't a great way to make serious money and most investors prefer better risk (smaller & lighter also).
Have some extra money and want to take a chance NIB pinball machines might be an OK risk. But once you consider space & the other risk I think there are much better ways to make money.