I would argue that the stagnation option is the most likely. After all, the exponential growth in technology and economics is made up of a series of smaller 'S' curves in individual technologies and industries. Each goes through an initial period of slow growth, followed by a rapid expansion, and then a slowing and leveling off of growth. We're already seeing this same pattern on a larger scale with population growth. Certainly a significant drop is possible. I suppose a 'singularity' is conceivable as well. But an S curve, with growth leveling off over time, seems like a good prior. (Of course, it won't be perfectly smooth. Zoomed in, it might end up feeling more like the sawtooth graph.)