Good analysis. As a counterexample, Slack quickly moved upmarket, transformed their product from a tool for small teams into a platform with enterprise-level features, and reached a multi-billion dollar valuation commensurate with the growth opportunity.
Agree it's a good analysis. What I wonder is why the fog creek people didn't pursue that path(building Trello into more of a platform) as opposed to taking the exit. I mean it appears that it was quite a good exit for them, but there not "starving artists" and they definitely have the chops to build the product out, so why not?