Any renders you saw are nothing official from Apple, they're literally meaningless.
As for your second point, in my opinion, you're mistaken. Customers do want a customizable module based Mac, it's what people on different creative forums have been clamoring for for years. Apple isn't just making up their own ideas on the Mac Pro, they've been contacting many professionals for opinions on it.
Tons of internal expansion isn't really that practical anymore with as fast as things like graphics cards are changing and the fact people want to take a workstation on the go if needed.
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Except they aren't on the outside. As fast as Thunderbolt 3 is, it loses anywhere between 10-as much s 35% of the performance an internal card is capable of. The fact that graphics cards change so quickly is EXACTLY the reason internal expansion is needed. I know that MOST of us don't go inside of our machines, but, I've been inside of iMacs, Mac Pro's and the like over the years, and I just feel that there HAS been a vocal number of pros who have begged for years for internal expansion over the years. Maybe Apple is willing to seed that market. I would hope not, but if I spend $1500 on a card, I want every last drop of performance that card is capable of.