So do you have the toolchain necessary to build new firmware for these games? Willing to pay $1000 (or more) for a compiler license? How will you compile in graphics and audio files that contain copyrighted/licensed material? Will you buy a license for the RTOS used in the code? How about other third-party, closed-source, licensed libraries that Stern might be using? Do you expect Stern and other manufacturers to populate a JTAG port on their production hardware so you can debug your code changes?

Are non-coders willing to pay more for their pinball machines so a small percentage of customers can do those things? How else will Stern pay the higher license fees to IP owners and to their staff and lawyers to make the changes necessary to release the source code? Do I really want Stern software engineers working to make Open Source releases instead of delivering new code? Reviewing every issue and pull request that comes into their repo?

What sort of leverage drives your demand? "Do it or the undersigned won't buy your games"? Will Stern volume increase by even 1% if you can get the source code to their next title?

This is embedded software. How many hardware products have voluntarily released their code as Open Source, as opposed to doing it because they based their code on existing Open Source code that required it? Why should pinball manufacturers be any different?