Well right now there isn't a food shortage so what should we do while there isn't a food shortage? Should we perhaps say, that we only jettison the disabled people once we're actually running out of food and are forced to make a tough decision about who to keep alive, instead of doing it 20 years in advance just because there might be a shortage one day?

Whether we as a society support those who cannot productively contribute is orthogonal to capitalism. Capitalism itself has nothing to say about what we do with the increases in productivity. You can see this in many developed countries, which have wealth and surplus thanks to their capitalistic markets, but they also have high taxes and a strong safety net.