Their thesis is clear and falsifiable, it's just that the timelines are very large. If you are going to criticize the thesis, it would make sense to address the actual arguments of ongoing cost-per-resident of various pieces of infrastructure, rather than just calling the organization 'biased'.
The Strong Towns "hypothesis" is at least partly falsified by objective economic data. Many suburbs/exurbs have been around for decades now. If the costs per resident were truly unsustainable then we would expect to see a surge in Chapter 9 bankruptcies, or at least a major spike in muni bond insurance premiums. So far that isn't happening. Most cities manage to muddle through and patch their infrastructure well enough to keep things working.
I mean, the main argument against their thesis is just that most people will pay more for services than they otherwise would have and be poorer, but not collapse. On the other hand, the trouble in the mid-west and rustbelt we are seeing more and more bailouts on infrastructure (the $1B Blatnik bridge fix, for example, will be paid for by the US Gov't), while also seeing more deferred maintenance.
The good news is that, again, this is falsifiable, and I'm an empiricist. If Phoenix doesn't can weather their future lack of growth without adding density, then I'll wave the white flag.
