I've been around this hobby for a long, long time. And here's what I know...
Williams did the "limited, once we're done we're done!" business model. This resulted in so many places waiting for close outs to happen so they could get their machines cheaper, which meant less money for the B/W bottom line. Eventually, B/W decided that to sort of 'stick it to' those people who did the waiting, they were going to take a great game and purposely underproduce it, selling it only to those who essentially bought immediately.
Viola, we have Medieval Madness, a game that was worth more than NIB the day that it came out, and a game that B/W could have made lots more of had they not had to figure against the "guess how many of these we will sell" market.
It didn't work. Ops were angry, so they started to buy less machines because clearly (they felt) Williams didn't really care what they wanted. This led to lower and lower sales, and more and more clearance blow out specials.
When Gary took over Stern, they set up the factory in such a way that you could send down a LOTR followed immediately by a TSPP and then go right back to LOTR with minimal problems. This was a HUGE breakthrough - Now, Stern could effectively listen to their customers (the operators) while not overbuilding machines and needing to blow them out. Ops wouldn't feel ripped off buying TSPP NIB for $3,295 because next month they would blow them out for $2,000, they would know that the $3,295 price was what it would be today, tomorrow, or in a year from now if the game proved popular enough to keep making them.
Without this ability, during the time right before IM, Stern absolutely would have gone belly up. They met the market for the product that they were providing.
Which, by the way, until Jack came along wasn't a collectible. Jack saw the ability for it to be a collectible and convinced Stern to run LOTR LE years later, a run that Stern wasn't so sure about - it was a risk remaking a game with minimal upgrades and selling it for a price higher than it was supposed to cost. Supposedly, something with that relationship fractured over that game, and Jack went off to start his own company. Stern heard about the LE thing and thought they could jump on that to get the money from it before Jack could and started doing the same thing, which worked great for them. They had some hiccups along the way (Avatar LE being announced months after the machine came out in Pro, for instance), but it's clearly added to their business model.
But the Pro line was never meant to be a collectible. The LE line? Sure. But the Pro models are supposed to be out there taking quarters and getting new players to see what the heck pinball is anyway. If they want to, I'm certain they could run an IM down the line followed by an ACDC Premium followed by a Mustang LE with no problems.
It's the best manufacturing method for mass amounts of pinball machines period, and without it we wouldn't be here talking about Stern pinball machines at all. Pros are not "collectibles" unless they are collectible in the sense that you want to have some to play.