As other posters have pointed out, innovation really did not stop in 1992. Pinball 2000 was actually a success, but Williams decided to pull the plug despite that. The Pinball 2000 platform was quite revolutionary - from how the player played the game, to how the game was designed, maintained, and even upgraded.

I don't think free play really had anything but an ancillary role in the decline of the hobby. It was likely the rise of video games, and specifically home consoles, that really hurt pinball. People played video games at home, and then played newer/better games in the arcade. Due to the cost a pinball machine in the home is a rarity to this day, and so the exposure it received was never the same. Kids learned Mario and Nintendo, they didn't learn the silver ball and flippers.

Along the lines of the article, the most "interesting" idea I heard in pinball circles was that clear coating playfields was a harbinger of doom. Williams started doing this and called it "Diamond Plate". It was designed to reduce the need for mylar to protect the playfield.

The premise is that the idea worked TOO well. Playfields on the machines could look like new forever. A machine's lifetime was extended, the impetus to replace the machine was lowered, and sales suffered.