Also consider that most big players in the game have very little incentive to reduce screen-time in a meaningful way. The more time spent on devices consuming media the more they sell from their App store.
Of course others could follow with similar features and this would remove the possible advantage Apple had, leaving them with no advantage and reduced app store revenue. Or if others don't follow, this might drive developers to other platforms.
Don't forget that for any device that a child owns, the likelihood is the parent is the customer, and it's their repeat purchases that probably make the biggest difference.