I really don't think there is a bubble or lack-there-of.
I am 36, a product of 1980's arcades. I LOVED the video games and didn't pay much attention to pinball at all. It wasn't until I was into my 30's that I truly discovered what pinball had to offer. As has been argued before, a lot of pinsiders and pinball enthusiasts, have children that not only enjoy playing our games, but actually take part in the hobby as well. As long as there are hobbyists, and excitement in the youth, the novelty and excitement will continue forever. I cannot begin to explain how many kids I saw at the MPE with glowing eyes at playing these machines; how many parents I overheard talking about buying one for the home. I occasionally bring a pin into my restaurant and kids who never have the opportunity to play one are mesmerized by the game.
Pinball is pure, pinball is nostalgic, pinball is great. Sure prices are high (esp on NIB and "A" list titles), but I don't think that will hurt the hobby. As other threads have stated, the flippers and the greed are what end up ruining potential futures. The pure pinball fans know one another and in my circle give each other first shot at the purchase of each others games at non-inflated pricing.
I don't think you will ever see the rebirth of the traditional arcade...but there are still many other avenues for kids/adults to play and learn. As long as these companies keep making the machines, people will continue to buy, and people will continue to be amazed.

Is there really a bubble? Is it limited to collectors? Didn't the METLE pins sell on the Metallica website easily? Weren't a lot of collectors disappointed in not being able to get them? Thankfully I got lucky with my LE - I will play the heck out of it and not bat an eyelash on how much it is "worth". Pinball (and it's themes) are transcending the collector elements for sure.