The $15 figure, if aimed at for 2025, is very close to what the minimum wage was when I was in high school and early college -- maybe a bit ahead, depending on the next several years of inflation.
As for the price of burgers, various groups have calculated that, and for MacDonalds an immediate change to $15/hr would raise the price of a Big Mac by a dime, the price of a full meal by under forty cents.
The economy has departed from supply and demand because giant corporations have realized that they don't have to keep wages advancing to keep up with inflation because they can shove the cost onto the government. Walmart even has packets they hand out to new employees with everything needed to apply for all possible government programs. If you want supply and demand to work, you should be demanding tht those corporations be billed for the cost of all that aid to their employees.
In 1968 I was hired at McDonald's at $2.00 per hour. That would be $15. per hour in today's money adjusting for inflation. $1.10 was minimum wage in 1968. Why then have wages not kept up? It is indeed because of supply and demand. In '68 the store I worked at was in a Detroit suburb, at that time they didn't hire blacks or women. It was a teen age/ young (white) man job. There weren't many of us available. At 16 I worked 40-45 hours per week and made from 600. to 675 per week in today's money. At $11.00 per hour that's only $440. per week today. But let's look at who is working for that pay today. It's not a 16 year old any longer. It's an adult and in many cases they have a family. Looking close you will find that the crew is much smaller than back in '68. On a Friday at lunch or dinner we had 6 people on window, 5 on grill, 2 on fries, 1 on shakes and a few floaters to take up the slack or fill in.
18-20 people was not uncommon. I know this because I was promoted to assistant manager and made out crew schedules and assignments. Today you will find maybe 6 people even during a rush hour.
So given the fact that the hiring base has expanded and is more inclusive and the employees are "stuck" in a low pay field that has need for fewer workers to maintain operations there is nee need to give raises or pay much more than minimum wages. In '68 when a guy turned 18 he could move on to a higher paying job at a factory. If he had worked and obtained a degree of any kind (even liberal arts) their was a job waiting. Due to high tech, automation and jobs going to cheap labor over seas the demand is not there any longer and many find themselves among the working poor.
How did we get here? Automation and high tech are pretty easy to understand, they have been taking the place of people for decades. But outsourcing? I can see why the party of big business and the wealthy might get on board.
After all it was a great cure for inflation (too many dollars chasing too few goods). Inflation was despised by the rich as it became difficult to make a return on money that kept up with it. Capitol gains and all that passive income sort of thing. If you had a million dollars and didn't find a return that paid more than inflation your poor portfolio shrunk and you became less wealthy. But why would the democrats go along with global economics? Don't they care about the poor worker? Well, it benefited the liberals to have a system that kept workers more dependent upon government for a supplement for their needs. Health care, food stamps just to name a few. This meant that people could vote for themselves largess from the federal government if the liberals were in power. So we have a class of working poor, stuck in low pay jobs. It is all due to supply and demand of goods and workers.
Automation can be off set by the number of workers needed to build "robots", how ever most are built overseas so that does little to help the American worker. High tech jobs are great, but we import workers who are willing to work for cheaper wages thus increasing supply and lessening the demand and stagnating wages. So we find ourselves at square one. cheap labor supplemented by government hand outs. But, where does our government get the money? Why they borrow it from the countries that supply the goods that are sold on our shores. We must not tax the top 10% why that is a terrible thing to do. Our economy is a house of cards, it's all on paper. We must transition back to making goods on our shores producing a dynamic economy with a demand for workers thus increasing their value and wages.