Nevertheless, third parties have created compatible replacements that are much cheaper.

They just don't need to include a SSD controller, since that is already built into the SOC.

Notice how you specifically chose the mac mini for your example, the easiest upgradable machine in the lineup.

iMacs and macbook pros on the other hand are significantly more difficult. Apple could easily rectify this but choose not to, so that you are more likely to buy their overpriced upsells on the initial purchase.