Hello Mark,
Your concerns are fully justified, and, in fact, shared by quite a few developers. You needn't fear for the Python 2.7 apps already deployed. 2020 is not as critical a date as may appear at first sight. Python 2.7 will get support for the benefit of existing Google App Engine (GAE) customers using Python 2.7 well beyond this date.
Everyone is equally aware of difficulties faced by those who migrate a Python 2.7 GAE application to the 3.x series. Quite a few tools and infrastructure adjustments will facilitate this task, so developers should not feel too concerned. A smooth transition is in everyone's interest.