In an ideal world, said backup qualification should be acquired before you set foot in a flight school. And the main reason is far more trivial than the possibility of a pandemic halting international travel. As a pilot, one will only last as long as their medical. Sadly, there are a lot of health conditions with which one can live a long and high-quality life but forget about flying commercially ever again.
As for the current situation and the ways and means to keep yourself afloat until aviation recovers again - I can imagine that it depends on one's individual circumstances. Many pilots possess other skills and qualifications. Many even practice them as a second job in their downtime and now that second job can temporarily become the main one. Many hold degrees which could help now. Some have access to vocational training, others don't. Someone has a good grasp of fixing cars, real estate or foreign languages, someone else doesn't. And that's what will, to a large extent, decide who will spend the next months or years stacking shelves and who will be doing something else.