Being presented with this tired "what if" scenario is something of a rite of passage for many vegans. So much so, that the vegan community has started to reclaim the trope and poke fun:
The deserted island is meant to represent veganism put to the ultimate test: If you are so isolated and limited in resources, would you, lacking any alternative, consider eating something that compromised your scruples? Well, for a large section of the human population, this is no thought experiment. Food security is a real-life, on-going, unrelenting emergency.
If we want to see a vegan world, we need to deal with capitalist exploitation, structural inequality, and environmental racism. We can't expect people living on a "desert island" to have the same access and resources those living on the "mainland" do. Instead of interrogating those people who are trapped on figurative islands, we need to start figuring out how to afford them the same advantages and freedoms we enjoy.