I think pre-2000 pinball machines will always have a big collector market.
You don't need a lot of young pinball players to keep collector interest alive in the future.
Personal nostalgia is only a very small part of what makes stuff "collectable".
Just think about how many things have a huge collector following but have no buyers with a personal history of using the item.
When a 1925 Duesenberg automobile sells for $500,000 at auction, do you think it's because the buyer was driving a Duesenberg around town with his dad when he was 14?
How about when a 1917 Luger sells for $5,000? Is that because the buyer belonged to the Disney Trench Warfare Fan Club and went to trench warfare camp in the summer?
The thing that makes collectable stuff collectable is the imagination of the buyers.
That imagination doesn't require any prior use of the item.
Pinball machines are like jukeboxes, they are A.) very cool and impressive looking and B.) a prime example of 20th century "Americana".
Pinball, jukeboxes, coke machines are all cool enough to generate a collector market spontaneously.
The thing that's going to eventually drop in value is the 80s arcade games. If somebody wasn't gleefully plunking quarters into the machines in 1982, then they seem to just shrug their shoulders at them. Arcade games aren't impressive enough at face value to spark any interest.
That's my 2 cents on it, YMMV