Basically just share what you love about vintage machines. Sincere enthusiasm is contagious. I agree there's no need to slam modern machines but you can stress that they're getting at least the same level of quality as a high end new machine for a small fraction of the cost.
There's other things you can do too like maybe putting a tag on the machine with needle and bobbin info so people will know what they need and that they're easy to get.
Many people are afraid of older machines (not just sewing machines) because there's no guarantee. Those people you probably won't be able to convince no matter what you do but there are plenty out there that do recognize good value.
Rodney