Well we don't have to ponder that question. There is actually a country we can look at to see how things would work: Japan.
So, how does it work in Japan?
People in Japan spend a larger percentage of their income on food than any other country in the developed world. The food is very high quality though and often very fresh.
There are a lot of small farms in Japan, and a lot of these farmers are older people. Plenty of elderly people out there in the fields. When asked, they say working out in the fields keeps their bodies healthy and able to keep moving. The country has a big problem with younger people moving to the cities looking for better opportunities, so it's left a lot of small villages with only old people.
In Japan agriculture is heavily subsidized by government. This has been a contentious economic policy at times, but there is strong support from the public for this policy to continue. Many people value the country's agricultural traditions and availability of high quality traditional foods. Farmer groups especially have a strong lobbying presence in the government. Agriculture provides an important source of income for the communities outside the more urbanized areas, so the problem of the rural areas becoming depopulated would probably be even worse if it were not for these subsidies. With employment problems for younger people the government even set up a work program to pay them to help older people on their farms.
Since Japanese people are the ones running the farms, there's a strong connection between the farms and the community, even including the people in the cities. Or at least that's the way the society likes to imagine it is. These perceptions are played up by corporate food producers and political leaders.
One of the other main reasons for the subsidies is that Japan values its food security and sees it as a national security issue. In the event of a war it would be important that the country could have a reliable food supply.
Agriculture in Japan is very important in Japan and is part of the reason the countryside looks so picturesque. Although most of the population in Japan lives in crowded cities without much open space, there is still a sense of fondness and longing for the agricultural past, with rice paddies, open fields, and green hills. In Japan the agricultural areas are only an hour or two away from the big cities and many people see agricultural fields outside the train windows on their daily hour-long commute to work. So this is still something that is of value to city dwellers in their daily lives, just being able to have a nice view out the window.
What about restaurants? Without foreign workers to work for low wages, how does that work? There are some very good restaurants in Japan with very high quality food. Most people in Japan cannot afford to go to these expensive restaurants very often. There are a lot of very small family owned shops run by husband and wife. Such little restaurants can only fit around 8 to 14 people. The prices are only a little more expensive than comparable Asian restaurants in high cost of living regions in the U.S. Noodle shops are also very popular among many people. These are designed for convenience and fast eating and, although most customers sit down, the restaurants encourage the customers to eat fast and not stay very long.
In Japanese noodle shops they try to quickly shoe customers out to make room for other customers coming in. This helps cut labor costs because the same employees can serve several times as many customers, with more customer turnover. Or they may offer substantial discounts if you come in to eat during the slower hours. To sum it up, it basically forces the customer to modify their behavior a little bit so each employee can serve more customers.
These establishments have low prices and are only a little more expensive than fast food in the U.S. Basically they are just noodles in broth with a little bit of meat and a tiny bit of vegetable.
Overall, the quality to price ratio isn't really that different from many parts of the U.S. Especially when one considers the high population density in Japan and the higher cost of restaurant space.
Many convenience stores carry small refrigerated fresh meal trays, with fresh seafood and vegetables, and can provide a small healthy lunch. These are prepared and supplied by outside vendors, and are reasonably priced. A comparable concept to this doesn't exist in the U.S. Containing perishable food, the meal trays have to be replaced every two to three days. So in a sense, the Japanese have much healthier fast food than America, though the portions may be smaller than what many Americans are used to. Rice and vegetables (combined with just a little bit of fatty fish) fill you up though.
So in conclusion the Japanese do not really suffer for lack of low wage labor in restaurants. They just have to be a little more efficient in their use of labor. And there is a fair level of poverty in Japan, but it's relative poverty, since the rents for housing space are high.
You'll also see a lot of teenagers and young adults doing these jobs, and in many cases semi-retired part-time working older people, even though they may be moving a little slow. These same old people might be considered too old to work in the food serving business if they were in the U.S.
Going off the subject of Japan and back to agricultural economics...
Something to realize, 95% of the sticker price of the cost of food comes just from getting it from farm to supermarket, through the distribution chain, and the cost of retail. This article elaborates on how, even though an orange can sell for $1 in a supermarket, the farmers themselves are only being paid about 5 cents for each orange.
http://nfwm.org/education-center/farm-w ... low-wages/
What that means is that paying agricultural workers more is not going to have a very big impact on end consumer prices.
There was something else interesting I came across.
I had always assumed that after the US Civil War, when slavery was ended, the cost of cotton must have gone up. But apparently that was not the case.
price of cotton before the start of the Civil War in 1860 was 10 cents a pound.
http://mshistorynow.mdah.state.ms.us/ar ... -civil-war
price of cotton in 1876 was 9.7 cents per pound.
It appears the burden of not having slaves must have probably fell entirely on the producers then, and not the consumer.
That is, if you take away the low cost labor it will mostly be the businesses that will suffer lower profits, rather than all of that going to higher prices for the consumer.
(might also make those agricultural fields worth less if the profit margins are lower)
related thread: Immigration Decimates Family Owned Farms in New Zealand
viewtopic.php?f=86&p=15087708#p15087708 (in the Australia section)
Basically these medium sized family farms are not able to compete with the cheap labor on big corporate farms, and the foreign migrant workers are also driven faster and harder than what previous workers in New Zealand would have been willing to do. Ironically, even though most of the work on family farms is done my middle aged older people, those people at their age would not have the required stamina to work on the corporate farms.