Whether or not you view them as investments, I think the question of whether or not they'll make good investments is an interesting topic for discussion.
I think arcades are certainly cheap right now, but so are cars from the 80's and 90's by comparison. Once you get into the 70's (pinballs and cameros) things start to get more expensive. Cars from the 60's and earlier tend to be worth more the older they get.
In order for an arcade to retain it's value, you would need to know how to keep it in good repair and a way to keep it away from vandals. When you consider how much money the world government's are printing, I think it's likely that anything rare/hard to get will see a rise in value (anyone seen the price of gas lately?) The higher inflation runs, the more we pay for arcades and parts parts. If you were a buy and hold hobbiest, I would be willing to bet a free lunch that ten years from now you'll see a better return on a perfect condition arcade game than you will in the S&P500.
On the other hand, if 3D printing allows home hobbiests to print parts as easy as it is for the Fed to print dollars, then a home collection could make a terrible investment over the long run. I think this means that it's quite possible that you're friends are right when they tell you that our games will move from vintage to antique values, but if you don't enjoy your games, than you'd be crazy to think of it as an investment because one funny bump or spilled drink or leap forward in technology and your stock could lost 80% of it's value.