In my view, the EM market is growing. As one of the administrators of the EM PINBALL Facebook page, I have witnessed in that group a robust membership growth. The group, which began in December 2014, currently has 3,764 members. In the last few months, membership has increased by 50+ per week and in some weeks twice that rate. Similar growth is evident with other Facebook groups, such as those dedicated to woodrails and vintage arcade/coin-op machines.
Whether the interest in EMs is a product of high NIB games and/or the ability to repair EMs without regard to esoteric part availability is certainly a valid debate topic.
Irrespective of underlying reasons for entry into the electromechanical game market, I have observed one constant in the EM marketplace. The low-production, scarce and desirable EM titles, in good condition, have historically survived significant price fluctuations. That class of game, with few exceptions, has increased in value over the last 30 years. Indeed, most have enjoyed substantial appreciation during each decade. Gottliebs have been the most predictable. Williams and Bally have been less so.
Back in the nineties, the woodrail market experienced a spike, primarily because the European collectors began buying up just about any available Gottlieb woodrail. In the years thereafter, the values of the lesser woodrail titles reverted to previous levels whereas the prized titles catapulted to new, considerably higher pricing floors.
The sixties EM market peaked in the last decade or earlier for restored games. However, the project game prices have increased. Thus, the sixties EM market is currently more truncated.
The seventies EM market has exhibited enhanced valuation, especially in the last 10 years. Arguably, there is some validity to the "nostalgia" postulate for the marked interest in seventies games in recent years.
The prewar pinball marketplace has skyrocketed in the last 7 - 12 years, especially on the most sought-after titles. It is self-evident that collectors/enthusiasts purchasing 1930s vintage games never played those games on location. Thus, the "nostalgia theory" is an untenable explanation for the demonstrable spike in prewar game value. Likewise, the "dying off/estate sale theory" has not manifested an influx of prewar games to the marketplace during the last 20 years. I suspect that cool mechanical items will always be cool and collectors of each generation will tend to recognize that fact. The growing pinball market has exposed new, younger enthusiasts to these gems.
I know of no empirical evidence to suggest, in the future, that the above historical pricing vector will waver on the scarce and well-maintained EM titles. EMs will never be remade. Retro Pinball's King of Diamonds remake was a solid state game, which only served to heighten interest and enhance the value of the EM counterpart. Benchmark's solid state remake of 1954 Genco's Two Player Basketball and 1957 Williams Ten Strike also increased the values of those classic EM arcade games. In contrast, the MM and AFM remakes had the opposite effect on the originals.