And they will immediately run into 1) ban-happy regulators who will probe them for monopolistic power (withholding something as crucial as WhatsApp or G-Suite from the main App Store is pretty suspect), and for data collection (everyone already suspects them anyway) and 2) burned-out, pissed-off consumers who are sick of juggling all of their user accounts and being jerked around by yet another platform.

I have yet to see a convincing argument that any of these corporations will be able to charm users into joining competing app stores. So instead they will try to force them. Leading to backlash, and within days you'd see them putting their apps back on the App Store and begging the public for forgiveness. These are not companies who are particularly good at delivering huge new products anymore, and the difficulty is compounded by the fact that they would be playing on Apple's own platform.