I have thought about this a lot. It is a nuanced discussion and I will be writing a sloppy answer. These are as of yet relatively unconnected lines of thought. Please bear with me.
I think that having broadly trained base of citizens is helpful in a whole heck of a lot of ways. Knowledge of basic threat management and trauma medicine should be learned by just about everyone with a brain.
The problem you comes to a fundamental fact of human psychology: you can lead a horse to water, but you can't make it drink. You can force the average person to take all the classes in the world, and they still won't learn anything. Case in point the absolute failure of the public school system.
Echoing what has been said before, military service doesn’t change people as much as the media says it does. Life in the service only really agrees with a select few. Like any other career, sure you can take any person off the street and put them through an extensive engineering course. Odds are, unless that person was already predisposed to that particular field of engineering, they are going to do significantly worse than someone who chose that career. That person doesn’t start magically thinking like an engineer either.
People only learn when they want to. People only toughen up when they want to. Besides, There are much less unwieldy way to change the cultural atmosphere than mass conscription. With the broad trend line of social tension ratcheting up over the past couple of decades, that atmosphere is already changing. More people are getting interested in self defense and obtaining some level of self responsibility.
We used to have strong communities here in the US, a long long time ago. Those communities had a local militia which was the town pride and joy... before sportsball teams became the town pride and joy. Which is not to say that those militias were the be all end all, in fact they were regarded as a sort of proto-gentlemen's club. To be used for social drinking and showing off. Still, a properly organized and trained militia can increase the resilience of the community from the blowing winds of any storm, Political or meteorological.
“Service guarantees citizenship” has a lot of structural issues. So does the Active (voting) and Passive (non-voting) sub variant of that system. The biggest of which is: The idea of everyone is a citizen and every citizen is a potential soldier is a powerful idea, and is how we justify the draft in the first place. Saying not all residents are citizens, or the same type of citizens, significantly weakens or destroys the manpower pool. The bigger issue is that is sets up a very obvious class divide and foments the growth of revolutionary leftists cough cough The French Revolution cough cough. Because the “under” class would almost always be bigger, or grow over time, it become harder to sustain that scheme.
Ideally though, A system should be set up in such a way that encouragement of self improvement. The problem is how to set up a sustainable arrangement.
So TL;DR Join your local not-a-complete-joke militia today.
