Conscription is bad, but being invaded is (usually) worse. It is the responsibility of the people to make sure their leaders are going to make appropriate decisions to safeguard the people in extremely difficult times, including when to force life-threatening collective action to avoid an even worse outcome.

Ideally, if you needed to conscript people, you'd test everyone for suitability. But if you have time to do stuff like that, you probably don't need conscription.

So you need some quick heuristics to make the sorting job easy. The consequences of slowness, or of failure, are liable to be horrific. So, what do you do? What quick heuristics can give you the right qualities?

(1) young or middle-aged, (2) male, and (3) not disabled. Ta-da! Easily verified, pretty good filter. Far from perfect, but you do what you need to.

If you were talking about Russia, I'd agree with you.

If you were arguing that women in their 20s (without dependent children) were a comparably good choice to men in their 50s (also without dependent children), I'd agree with you--I think taking the time to get that detail right is time that can be afforded, even if capability-testing can't, really.

But I don't think your analysis properly captures the reality of the situation.

"I don't want to fight and die to protect my country, but I'll do it if everyone has to" is a perfectly reasonable attitude. We don't make taxes voluntary. But most people pay them willingly precisely because it's universal and involuntary. And most countries have an income tax that's graded by income as a heuristic to help put the greater share of the burden on those who presumably are more able to bear it. It's really not that different. It's just way more extreme.