Simple, apple will have to provide the same benefit otherwise I guarantee there will be complaints of uncompetitive behavior.
It's a bit unclear to me what exact position the person you were replying to holds,
and what specific "benefit" you're referring to, but it
seems to be along the lines of:
"Apps purchased from third-party app stores by one person should also become available to their family members through Apple's Family Sharing"
Do clarify if that's not what you're talking about. If that
is the case you two are making, I'd disagree. From Apple's
Family Sharing page:
"Family Sharing lets you and up to five other family members share access to amazing Apple services like Apple Music, Apple TV+, Apple News+, Apple Arcade, and Apple Card. Your group can also share iTunes, Apple Books, and App Store purchases, an iCloud storage plan, and a family photo album."
These are services offered by
Apple, some of which give developers the
option to allow Family Sharing in their products made available through those services
. It's a value-add for doing business with Apple. See
Darkroom, for example - available on the App Store on iOS, iPadOS and macOS, with Family Sharing. If you take your business to other parties I don't see how Apple would be required to add value to their services, too, outside of an actual business deal between Apple and that third party.
Even now if you purchase something through Steam on macOS, Apple's Family Sharing does not apply to it because Steam is not an Apple service and no business deal exists between them. You do have access to
Steam Family Sharing, however, and that feature
might extend to an iOS / iPadOS Steam Store if such would become available in the future. Steam does not, however, make purchases on their store available to family members using the Epic Store for example.