Here are a couple of things that come to mind:
1. Have one set up somewhere with the lid just loose, so you can take it off and show people the metal.
2. Definitely show them the machine doing something. A project in progress is a great idea - just don't be too attached to the project because when you sell the machine with the project half done your 1/4" WILL change on the next machine you move the project to.
3. Adjusted for today's dollars, many of the machines that we're talking about would have been valued in the several thousands of dollars. The first Singers in today's money would be worth over $40K new - or roughly a year's wages for a lot of the population. Even the postWW2 machines often cost into what today would be the 5 figure mark for straight stitch only machines. You simply can't compare that quality with a sub$1k machine these days.
That often helps put it in perspective.
That said, some people just won't change their minds and it's really not for you to change it for them if they don't want to. It's easier to "change" the minds of someone who wants your expertise.