I don't think the extra 100 is the issue, it's the timing.
I don't imagine people buying an LE when it was X/500 would not have bought if it had been announced as X/600, or possibly even X/1000. But, crucially, selling something as limited, taking orders and money for it and then going "we could make some more money here, let's add another 100" is disrespectful of the existing LE buyers, and unprofessional.
Also I don't necessarily buy that Stern could sell an unlimited amount of LEs. The reason many people will buy a LE, at the inflated prices, is because it is sold as limited. The investment potential might not be at the forefront of customer's minds, but it would be a factor, and they would certainly be thinking that it ought to retain its value more. A $10,000 pin that pretty much anyone can get because supply exists to meet demand would simply mean that the value would depreciate in line with the non-limited editions. Limited edition as a concept only works as a value proposition when demand outstrips supply, and where the manufacturer understands and accepts that they could sell more but choose not to in order to create exclusivity. As was said earlier by another member - if they sold it as "trim level 3" it wouldn't have the same appeal. People would quite rightly just compare what you actually get for your extra cash over "trim level 2", and it becomes a different value proposition entirely.
Ultimately though the issue is of trust and professionalism I think. If Stern are prepared to change the parameters of LEs after the fact, when the order books are already open, there's no reason to trust that they wouldn't do it again, or even decide to do something else - more impactful - that would impact the value proposition or exclusivity of a LE purchase.