This decision benefits absolutely no one, and inconveniences many. Not a single player will be better off for losing the option to turn off Exp. Share, so why are people framing this as a positive change?
Gamefreak has never balanced their games to account for people using or not using Exp. Share, so this does nothing to improve the balance or reduce the balancing workload.
Exp. Share Off was already an option in Gens 6 and 7. It's not a new feature that Gamefreak would need to research, develop, and implement.
These games aren't made exclusively for children, so whether children would care about this feature or not shouldn't be the only metric for whether it's worth including. The games have featured adult actors in their advertisement; ad copy has included phrases like "Whether you started your journey on Game Boy, Nintendo Switch, or anything in between"; the official tournaments have a Masters division which includes and is frequently won by adults; the EV, IV, and breeding systems were certainly never designed to be child-friendly, yet Gamefreak continues to advance them.
Pokemon already includes a second difficulty option in the Shift/Set battle style toggle. The Shift battle style is much easier than Set, and is enabled by default. If this feature can stay, why not Exp. Share Off?
It's irrelevant what other RPGs do because this is a conversation about the Pokemon series, and direct comparisons between differing battle systems will always be flawed. This isn't "following the standard set by other RPGs"; it's removing a traditional feature of the series.
Ohmori's understanding of why players choose to turn off Exp. Share is, based on his quote, incomplete. I've yet to meet anyone who turned it off for the sake of powerleveling one Pokemon and no others. The motivation behind it is maintaining access to a full team while reducing Exp. intake, keeping the party underleveled and making the game more challenging. Putting all other Pokemon in the PC to simulate Exp. Share Off is a terrible solution, if it can even be called that, because you no longer have access to your full party in battle, and it would be deeply inconvenient as well.
There are trade-offs to cutting the Dex, and it was a decision made for understandable reasons, even if not everyone would agree with those reasons, the consequences of the cuts, or the implementation. That's not the case here.