Simple….it ends up mostly nuking the cottage industry of third-party options and helps bolster their subscription iCloud plan offering ...
Respectfully, I disagree. I'm an Apple One subscriber, and Apple recently sent me a poll asking about my use of the service -- except that, they weren't
really interested in my assessment of the overall Apple One subscription, so much as they were laser focused specifically on my use of the Apple Arcade subset of the subscription. Thing is, while I do play some of the games, (because it's there!) Arcade was by no means a deciding factor to me subscribing in the first place -- nor will it be a deciding factor if I cancel my subscription. Other services drove my decision to subscribe, and Arcade is really just not that important to me.
To me, this "Invites" app falls into much the same category. In fact, perhaps it falls into an even deeper pit of obscurity, as I could actually conceive of some people disagreeing with me about Arcade's value -- and unlike Arcade, I really can't even see myself going with the "because it's there!" justification for using Invites.
I just don't see how Invites is going to attract much to Apple's subscription services at all -- other than controversy. Rather, shoehorning Invites into a subscription service alongside other (arguably more useful) stuff is going to greatly minimize it's impact on that cottage industry* as you put it, because the type of person who is
looking for that type of app isn't going to go subscribe to iCloud+ just to get it... they're going to do a quick search either via Google or the App Store, and use whatever free tool happens to look the most interesting, or whatever commercial tool looks the most flashy -- depending upon their personality.
* Invitations have existed since time immemorial... it's anything but a "cottage industry". Apple may or may not have copied their design from one specific app in their App Store... but they most certainly are not going to have any significant impact on the larger market for this type of service, digital or otherwise.