To answer the original question: I think cgc customers would buy a new software version if it's similar to buying the color dmd.
For a few $100, install a new chip and you're ready to go.

Me personally, I don't have a cgc game but would be interested if I had.

I have my Dialed In and love it - mostly for the shots / playfield, not so much for the theme and sounds..
I'd buy new software for it (if it's a complete new theme, fully executed).
I hope that someone will get MPF working with JJP games and then I can try to make something myself

Same for my ACNC. I don't plan to ever sell it, if I get tired of it in a few years I'll probably install a rpi in it.
(I'm a bit hesitant to change the assets on the installed computer as the Spooky install procedure doesn't install a full image).

I think the interest for new software is bigger with people who own recent released games and are new in the hobby.
Those +10 years in the hobby (and more often have wpc games) are less interested in it, or in paying for it. They'll probably want something like Sorens fixes. They are not waiting for a total new ruleset as they love the games they have for what they are.

As for older games - all the comments made here are true.
It requires a lot of skills to create a fully finished gamerom, something not many people all posses.
And if you do it's mostly a labor of love, because you want to do it yourself.