I'm not sure about that. Maybe? But... Firstly, there are surprisingly many people who are insanely patriotic so would volunteer anyway (perhaps fewer than in the past but perhaps still enough; see point three). Secondly, there are surprisingly many people who enjoy violence and killing people so would volunteer anyway (this probably hasn't changed). Thirdly, modern warfare doesn't need large numbers of people (this has definitely changed over time). And fourthly, a lot of modern people rather object to being ordered around by the government (I think this has probably increased a bit, at least; I can imagine that there are even people who would volunteer for military service when it's optional but would resist being conscripted).