So, to me, this is absolutely bonkers.
IPS' team has noticed that their plugin community is a fraction of what it used to be, thinks this is because everyone has switched to bespoke work (You sure it's not the other way around? People using bespoke work /because/ the plugin community has a fraction of the size it did back when there was a free version and before IPS was trying to be everything for every person by default?).
So they /murder/ pre-made plugins. No, we're not going to see thriving new marketplaces. Excited devs will start some, none of these will be as active as the current one, most if not all of these efforts will peter out quickly. Not having the marketplace in the panel, not having them vetted by IPS, not having people browse over them prior to purchasing IPS (and plan out a setup with plugins), not having their plugins listed on IPS' websites are all small cuts that together, cut deep. IPS' website was never a limiter on other marketplaces (they've always existed for themes and some plugins), but free advertisement to those who did have those. You're merely taking away an option and your stamp of approval.
And you do this brilliant move in the middle of a version, announcing it to your costumers roughly a month up-front.
Definitely not obvious you're springing it this way to ensure customers don't either, mass-hug a former major version to keep their plugins, or start planning migrations away from IPS between the time of announcement and release.
Now, months after it happened, I google 'Invisionpower plugins marketplace' and find nothing on the first page.
Sincerely, as someone who was a fan once of IPS, you people suck.