I remember CMYK being the most common complaint (after the UI) 20 years ago, too. And at the time the entire GIMP project insisted that you never needed it and RGB was good enough. By coders for coders, indeed.
Out of curiosity, what improvements in 3.0 RC are you skeptical of? If you're willing to share, it'd be helpful feedback for us. :)
rgb is a larger color space than cmyk, that is to say the colors that cmyk can represent are a subset of rgb.
You are doing the work on an rgb monitor. what does having the illusion that you are working in cmyk get you? just use a cmyk color picker
but most damning, industrial printers are not CMYK, they are spot color, that is, they will be filled with essentially random primary colors chosen to best fit the final print. When you do need very fine control over the final color, you don't want a cmyk aware system you want a spot color aware system. And even then, you will only get close, requiring many test prints to validate the final result.
You're not going to argue and discuss the merit of such and such color spaces with them; you're going to switch tools so that you get them to print your wedding invitations.