The problem doesn't ever go away - if you're using Mailboxapp, you're forever going to be vulnerable; you're free to use them and accept that risk, of course, but it's still a security problem with that service.

There is no such thing as 'sufficient trust estabilished' - trusting Mailboxapp right now doesn't in any way imply that it will be trustworthy enough for your needs (however large or small) after, say, a year. If you're using software X, for example, then you can think about renewing trust only when going to software X+1; but with such a service they can go from 'doing only good things' to 'intentially selling you out' at any arbitrary time.

For example, look at what's happening at Buffer. By using intro or Mailboxapp, you've just added another company whose decisions may screw you up, and that is a problem.