A time traveller cannot just invest money and wait. Because time travel changes everything! Here are some examples:

When time travel becomes possible, people have more choices. So the world changes more quickly. Don't think you can keep a secret like that for long! If you invest money then travel to the future, how do you know they will still use money? Or that won't outlaw inequality? Or jail you for cheating? What if your actions create alternate time lines and you are lost forever? With time travel all our assumptions have to be examined: the only certainty is change!

Should I build a model time machine and show them? Or should I stop trying to persuade them, and build a full size machine and use it myself? Or do something else?