I think one of the responses, is that the "here today, gone tomorrow" train of thought can apply to anything. But to provide some reassurance, one of the things you can look at is the community - a good size and decent amount of recent activity are a good indicator that the project is "alive".
Another is to read into the reasons why it came into existence in the first place, and how/why it still exists today - which I believe Ryan has covered both on the website, and quoted in several places on the forum (paraphrasing - "the project is funded by the paid-for modules and by people paying to have websites built using it").
The fact that it is open source is surely another winning point. The code is out there now - there is no way to retract what's already been published, it can always be forked and maintained by any interested party. And it's PHP and MySQL - there are no shortage of developers with those skills.
Several of the things mentioned above are also true of WordPress (and others!). I do believe that some people and organisations hear about these "wonderful" open source CMSs (WordPress, Drupal) and use those as the baseline to judge anything else against. But as we know, those tools aren't always necessarily up to what they promise.
Perhaps a collection of proper case studies of sites built with ProcessWire, involving the developers/clients, would be a great help in this area. Particularly if some were "high profile" to grab people's attention.