Apple will use it's dominant position to create lock in like how they did with iMessage instead of cooperating with other platforms on a common standard.
Oder friends and family are surprised when they want to video call over Facetime and find it hard to believe other people's phones don't have Apple apps.
Sounds like there’s no Apple walled garden lock-in for recipients of the Invites? Only for creating/managing them?
When I think of people I know who have iCloud+ subscriptions, it's mostly people who thought they needed them because of the scary notifications their phones sent them about running out of their initial 5GB of storage (which of course is not enough for any modern phone).
These are not people who are going to download and figure out how to use a new invite app. They are going to keep using evite just like they always have.
There are some people who may use iCloud+ or a bundle because they like the fitness features or have a family that makes it make sense. I probably know some people like this. But I have never had a conversation with anyone about iCloud+, ever in my life. Only dealt with questions from non-tech-savvy family members who were scared about notifications that they were running out of space and needed to "upgrade" to this paid service.
If you don't want to use it to create invitations, don't. There's zero requirement for you to have an account if others invite you to something, and it sounds kind of preposterous to complain about other people choosing to pay for a service that you can then participate with for free.
And it's obviously not likely to get anyone over the pay wall to buy iCloud+. This invitation feature has many free competitors. I don't begrudge Apple for creating this, but I won't be surprised if I keep getting evites and paperless post invites, and never an Apple Invite or whatever it's called.