On the contrary, a volunteer army allows the ruling class to prosecute wars without risk to their own families.

As does conscription, since there are always exceptions made for that explicit purpose.

So that works out the same either way.

If a war arrives that is necessary, justified, and also has broad support among the population there will still be those who avoid fighting because they know that others will do so for them.

Yes - there will always be such people. The issue is how many of them there would be.

I would say that a nation that's unhealthy enough to have so many such people that they would make the difference between winning and losing deserves to lose.

You can make a similar argument about taxation. By your logic payment should be optional, since a society that genuinely wants to be just and fair should also voluntarily want to give money to achieve that.

Yes, and I in fact would. And with the same proviso - any society that would fail as a result deserves to fail.