I don't really have a dog in this fight, but I am opinionated. Please feel free to accuse me of disgracing my ancestors with the comments that follow . . . . .
There is a lot that has been tested by BIG SOFTWARE in the area of piracy. However, the reigns usually loosen on older software in pretty much every area of software distribution, and when you treat your customers like criminals, even the "good ones" tend to trend towards the dark side.
I'm not going to claim to be any kind of expert in homebrew classic gaming software. If you say releasing the ROM will devalue your product then I have no real reason to disagree, but it sounds like you know your market and how many sales that market can support. What is the point of holding back the ROM after the sales are completed? retroillucid, I guess you are saying that you do release them so that question is for someone else.
Do developers and publishers hold back copies and offer them for auction later to make a bigger profit? That would make sense - why not release the ROM after that? I'm seriously asking because I can't really think of other reasons to hold onto the ROM.
SIDEBAR
Video game franchises tend to get around the problem of competing with their own discounted older games by releasing story driven sequels. It might be cool to see a planned series of games that would compel the player to purchase the next release. Then you could release your ROM for the previous game knowing that it would potentially drive more sales of the next. If you wanted, you could simply not release the final ROM. I'm just thinking and typing here, but that is basic product marketing stuff - make them want more.