This may be an ideal solution for you but its really not for most people.
A good example of this is that there were scams that involved having people paste some script into their chrome devtools and steal data. This worked fairly effectively. Facebook ended up doing some magic to show a warning message in the devtools console to tell people that no, you really shouldn't paste random stuff here, it will do bad things.
Configurability does come with a cost. And "the ability to reroute all push notifications through an arbitrary MITM" is a security cost that I expect wouldn't be worth it.
But I actually think you are mis-portraying the situation is because I don't think the average adult has the ability to comprehend these things. It's not the difference between adults and children, but adults and expert adults.
Here are some analogies to other parts of society that we don't have a problem with:
Seat belt laws
Hard hat required construction area
Safety guideline in Handling of hazardous materials
We basically said society doesn't trust you to decide for yourself about your safety - you just have to follow the rule.
Part of me wants to challenge the existing groupings of adults and children where all that matter is if you have been around the sun 18 times or not. This is but one weakness in the existing grouping that furthers my questions, such as why do we allow the non-expert adults a vote over laws, but deny a 17 year old the same?
I think if you take the groups of children, adults, and expert adults you will find upon reducing them to only two groups based on similarity that you are left with experts and non-experts.
The impulse to protect people from themselves is a dangerous one. In the article itself we see that in practice it is used to push inescapable spyware.
"But our spyware is better than their spyware!"
Google says they will protect you. But the truth is they are just concern trolling to shut down marginally worse competitors.
For kids and elderly that can't make decisions on their own it could be default -locked to some entity contractually bound to good ior. Locked with an administrator password. That would be a reasonable compromise.
But yeah, if you want to avoid Google's servers, then it's not enough. But in that case, you're probably on Google-free LineageOS anyway right?