See the power outage in Iberia as a counterpoint.

Also when there's a fiber cut, it usually takes out everyone since there are frequently shared conduits or poles.

Everyone reverting to mobile usually takes everyone out.

Wouldn't a widespread ground-based infrastructure outage also take out (or at least severely degrade) Starlink in the affected region if people were to widely use it as a backup solution?

Starlink has been known to carry traffic over lasers from Southern Africa to Europe and from New Zealand to Eastern USA. During the power outage in Spain/Portugal they proactively moved traffic to the UK

Local failures don't matter unless your country doesn't allow landing user traffic in other countries (Indonesia, Bangladesh)