Or at least it's not engaging with the obvious counterargument at all - that: "You may not need the scale now, but you may need it later". For a startup being a unicorn with a bajillion users is the only outcome that actually counts as success. It's the outcome they sell to their investors.
So sure, you can make a unscalable solution that works for the current moment. Most likely you wont need more. But that's only true b/c most startups don't end up unicorns. Most likely is you burn through their VC funding and fold
Okay stack overflow allegedly runs on a toaster, but most products don't fit that mold - and now that they're tied to their toaster it probably severely constrains what SO can do it terms of evolving their service