Thunderbolt drives are almost native speed so unless whatever work you are doing is heavily IO bound, external drives are cheap way to boost storage for most people.
Sure, Macs should not be so restricted but unless government take action, that's not going to happen.
Why are you mentioning external SSDs? We were talking about how difficult Apple makes it to swap internal drives.
External drives are not a replacement for internal drives. I have a laptop for a reason, and on crowded trains and planes needing some desk space to plug in dongles, huibs, and external drives just doesnt cut it.
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.