but why would older game prices go down?

Those are:
#1 already cheaper games
if people need money due to a recession then they are selling the most expensive ones and modern ones first
#2 already very limited and nearly dried up in colelctions
i.e. supply is limited and deals are largely gone
#3 they aint making more of them and more people want them.

If a recession happens then the games that will take a beating are everything 5k and greater that is less than 5 years old.
Supply is high, cost is high, and people will try to sell the easiest thing that they can get rid of the quickest (i.e. newer games)

pretty spot on.

Do it for the fun. Keep your games well maintained (plan on it being a good amount of time to do this) and eventually they will be paid off.
It all depends on location of course.

Pinball brings people in and keeps them at a bar, but it rarely makes money from a business standpoint.

You still are buying 5500+ games, paying sales/use tax for the game, paying local license fees, paying local sales on every quarter that comes in, paying for maintenance and your time, paying for insurance, paying for other business fees, etc...

Even with a great location and a great split, it takes a long time and lots of energy to make something beyond the fun pay off.

The one personal financial benefit I see is the ability to write off pinball business stuff. That is still not worth it financially.

If I were to do it in the future as a real business, I think the best model would be to try and become a distributor for Stern and Spooky and JJP. Then you get distributor pricing for games and can use your bar to also act as a sales floor. You can likely avoid sales/use on the actual machines if they are "floor models" and then you can make some decent money off selling games I would think.

Don't get me wrong, hobby operating is very rewarding for me. However, I enjoy learning and the challenges associated with it.
The player side benefit for me is that we get to buy games I would NEVER be able to afford or buy personally. It is pretty cool to have a BKSoR right now to explore. I can't readily afford any NIB game, but it is easier to justify for route.