It's also possible that things that grow 50x tend to be newer, meaning less gradual accretion of cruft in the code. Being newer also means more remaining instit

It's also possible that things that grow 50x tend to be newer, meaning less gradual accretion of cruft in the code. Being newer also means more remaining institutional knowledge in the organization about the technology. For 50x growth, the new thing we are thinking of rewriting probably also has some simple core purpose (like some kind of social network), making it easier to grok the existing code and indeed to rewrite it. By contrast a decrepit old payroll system will not have the growth dynamic, and will not be at all simple to rewrite - the code will be packed with small fixes and strange little features that are actually critically important, and that it would be disastrous to omit in a rewrite. These tie in with your last point.