No company would hesitate to fire every one of their employees if they could get machines to do better work for cheaper. The only reason McDonald's hasn't done it yet is because they haven't found a cost-effective way to do it.
Machines are great, but we do need to mind the way we use them.
In the short term the less intelligent half will be massively unemployed until we get something like Gattaca going and then everyone can basically be researchers (or writers, etc).
The real question is what happens after the singularity.
Probably more true than you realize. The most likely scenario for a technological singularity is an AI that writes a slightly more efficient AI, and so on until world domination if it's quick enough (which is likely).
So we better know exactly¹ what we want the AI to do, or it might for instance tile the solar system with molecular smileys to maximize a badly programmed notion of "happiness".
[1] With mathematical precision, no less.
But even then, that is only a hairs step away from 3d molecular printers just building amino acids and proteins up into the food you want anyway. Significantly fewer moving parts and things to break, too. I hope that happens in our lifetimes.
IMHO, "printing" food is a dream that will never happen. Much like flying cars. It sounds futuristic and all, but you just can't do it both cheaper (and better) than dropping a seed into the ground and waiting.
But we shall see, I guess :)
Another company(forgot it's name) is working on algae that manufactures sugar as an input for the biofuel industry. Their target is 4x cost reduction vs plant grown sugar.
If they succeed it would be interesting to see the dirsuption to the food industry.
Meh, the status quo will just spend their tremendous capital on a hate campaign to paint algae grown proteins as artificial and bad because they aren't bloody cow butt.
Inability to anticipate load is a problem here, but it's not as dire as you're claiming -- the worst is that someone waits for a while longer, and this only applies if you don't have humans manning the registers -- after all, humans can indeed issue an order to a robot of "give me a dozen macs now!"; that is an order which robots can accept. I'm much more concerned with the fact that a totally-automated burger joint might not notice a hole in the wall forming due to water damage, or a rat making its way in through the aforementioned hole in the wall, following the smells of delicious burgers.