depends on what 'we' is defined as, and what 'sustainability' is defined as...It's 2050: Can We Feed Nine Billion People Sustainably?
either way, food production is very efficient & there is no shortage of food.
nor will there be...
distribution of food is different consideration, altogether...
humans never distribute anything 'evenly'... or 'fairly'...
maybe termed 'cane & able' syndrome, w/ animals.
example... large child, small child... large child requires more calories per day to 'sustain' physical form. small child requires less. thus, larger child eats more.
this phenomenon is easily observed in humans, eagles & most organisms... parents feed most vigorous most...
just as gardeners select best cultivars & eliminate rest... crude, but still fact.
there are very fertile ares of planet where only ornamental plants grown, & shipped across globe, for beauty only... while native gardners maybe dont eat as well as would if food crops were grown & distributed to population, instead of specialty crop...
so... foo production not problem... if really got bad, those w/ skill (commercial members of these boards) would simply start growing food instead of other crops...
after all, gardening skill makes growing assorted crops possible... so, definitely, plenty that know how to grow own food & supply it, too.
cost of food different matter, as well...
main concern, for humanity, it seems, not so much food, as clean water...
once clean water private, or not clean, in general... real issues come.
easy enough to go back to agrarian lifestyle... not easy if all water on planet contaminated, or 'closely held'...
humans can live on small calorie diet (even just insects, roots, grains, vegetables) but not on dirty water... something to consider if doing 10-20% 'waste' in any system...
it is not efficiency of production, or even lack of surplus that makes food not available to some... it is costs built into markets to 'sustain' 'food chain'... define those terms for own...
enjoy your garden!