but only those who submit to master government and love big brother get to vote
Umm no, you can read the book, and it is fiction but you are equating service to your nation/or fellow citizens as being support for a tyrannical system.
In the book EVERYONE who serves starts out as a private or the civil service equivalent. Also, the service is really a sacrifice. Imagine a 75% dropout rate and a 5% mortality rate- in peacetime from the training. So there is no political overclass. In fact, many of the wealthier citizens tend to look down upon serving and voting (as they would never stoop to something as base as service).
Personally, I am not advocating it- but some aspects of it have merit. I believe a better start would be no voting rights if you rely on government assistance for basic needs or if you pay no taxes. Lifetime exemptions given to veterans or civil servants who served in hazardous duty.
Also, interestingly, Heinlein had the civil service side have positions available for anyone with the mental capacity to take an oath. (You’re a paraplegic with a heart condition and blind but you really want to be able to vote? OK, we think it is a bad idea but we will figure out some kind of semi hazardous duty you can perform in order to earn the right.
The thing you have to remember is-Heinlein wasn’t describing a new political system, He was telling a story about an entirely different society with different values from our own. Our society does not currently have enough altruism in it to consider such a system.