I honestly don't get the uproar here. This is opt-in, so how is it any more outrageous than existing MDM software that people opt-in to for work reasons which are way more invasive?
So the very fact that it is opt-in means it doesn't even try to "protect the children"; it just uses the children argument to promote the idea of total control, which will be then used for a host of other purposes.
technically you could not update to ios 15 or disable icloud, but it's not opt-in by any means.