I can't speak to anyone else, but it seems to be working well enough in our town. The overwhelming majority of kids don't have cell phones until high school. That doesn't mean your kids won't beg you for a smartphone, it just means you can say "no" without socially isolating them.
> As soon as a couple of kids have a smartphone
The point is to engage in collective action early enough that you can prevent these situations in the first place. Once a critical mass of kids have smartphones and their socialization and coordination moves to online spaces it becomes intensely isolating to be the only kid in a friend group without a smartphone.
True, but it is significantly easier for a single motivated individual to pull it off in their own community than for them to change laws at the territory/state or country/federal level.
And even if your end goal is a universal law, it usually makes sense to start at the lowest level from a process perspective. Laws at the community level are often a precursor to laws at the territory/state level, just as territory/state laws can be a precursor to country/federal laws. In the USA, both alcohol and age of consent laws have gone through (and in some respects are still going through) this process. Bypassing this (admittedly slow) laboratory of democracy to implement novel laws directly at the country/federal level results in controversial laws that often have unintended consequences.
> It’s way more sensible to just make it law.
Centralizing laws can be sensible if you're somewhat confident that the people writing and enforcing those laws will behave sensibly.
If you don't have that you get the rules destroyed by demanding parents bullying administrators and school boards.
True, but that's why you don't do it alone. You need to talk with other parents and encourage them to talk to others until the majority of parents understand the risks, support school phone bans, and let the administration, school board, and teachers know that they have your support.
If none of your child's friends and classmates have cell phones yet, I'd strongly encourage establishing a smartphone pact with the other parents. Our community used http://waituntil8th.org pledges but even a shared spreadsheet would work.