MTG is another flagship product by the same company, and WotC has copied aspects of that model many times in the past. It’s silly to assume that they can’t or won’t again. It’s hardly conspiracy to speculate that they might use the same digital paywall tactics in Sigil as they’ve used in Arena or MTGO.
WotC will have monopoly control over Sigil, even if they allow other systems to sell on their platform. That’s beyond dispute unless you can show me news of Kobold Press buying an ownership stake in this platform.
Sigil will be immediately competitive with other VTTs just because of the D&D brand, even if the software is trash. (No reason to assume it’ll be trash either.) Hasbro is spending a lot of money to make a Roll20 killer here.
You even admit that there will be lots of other revenue opportunities within Sigil, so why are you so adamant that they would never charge for multiple copies of 3D minis? You’re so sure that they would never that you won’t admit that they definitely could.