You can't 'prove' anything in sociology. Just framing things like that marginalizes your movement. Don't do it, let it go, and focus on things you can actually get people interested in. Otherwise you're relegated to the moon - that's the whole point here.
Sound Transit spends money collected on the eastside only on the eastside, and money collected in Seattle only in Seattle. The requirement is called 'subarea equity' - and it's why the eastside has about $1B in the bank today to be spent on East Link.
Neither the original Sound Move taxes nor the ST2 taxes could pay for East Link on their own. Waiting and saving up is the only way to pay for the project.