I'm not in support of trying to control what people do. And I'm guessing those for it tend to be in support if they believe they won't be subjected to it themselves, but would suddenly shift if they found themselves under the same policy.

That's how it usually seems to go. Individuals thinking they are above the policy and in a special class. Believing it's others that need controlling and not themselves.

If this can indeed be a mandate instead of just "demand parental controls", the issue would be inflexibility. For a lot of people, limit would be enough. But for a lot, who are using their phones for actually beneficial purposes, it is not, and such people (who can also be talented and useful) are completely disregarded by universal rules like this. I myself firmly believe that smartphones are not suitable for actual work, but every situation is individual, for a lot it might be genuinely necessary. Also maybe things like LineageOS could bypass this anyway.