If you could get a Mac mini with all components ready to be changed for 400-500$, it would sell like HOT CAKES!
Probably not. Most people really interested in a highly modular computer aren’t interested in a Mac, even a modular one. They’ve already got Linux and Windows project boxes to work on and configure to their hearts’ content. Second, there’s only so much change of components that can be done in a computer of the Mac mini’s form factor. Third, the PC market is pretty saturated these days, no model sells like hot cakes. If the Mac mini makes about 1% of Apple’s sales and Apple controls about 10% of the computer market, that makes the Mac mini about 0.1% of all computers sold. That probably qualifies it as one of the most popular individual models of computer (probably more popular than any one make or model of Dell computer, for instance). Even if there were high demand for a modular Mac mini, I doubt it would grow sales that substantially.