Decision-making in the armed forces is always performed by a "warrior class". The people who operate heavy war machinery are also career soldiers; conscripts are light infantry.

And there is a very real economic cost to mandatory military service. It only makes sense in the context of a small country (in terms of population) bordering a large aggressive neighbor, such as Finland or (possibly) Canada.

No, conscripts operate also tanks and artillery. They also load and prepare combat aircraft.

Artillery and tanks will have some kind of professional officer though.

Armed forces culture is also incredibly important. Goal/mission oriented organizations are harder to co-opt than top-down command structure organizations.