The need for widgets doesn't hinge on price, it hinges on availability. I suppose for some meta-types of widgets there's plasticity in the form of widget (like food; if carrots lose, then something picks up the slack because a guy's gotta eat). Similarly, if ford loses out on car sales, Honda picks them up because a guy's gotta get to work.
If cars cost more, people are more likely to hold on to their old cars longer or use public transportion, and sales decrease across the board.
Not if the increase in costs is matched by a corresponding increase in income. This is actually the reason why inflation (theoretically) doesn't destroy the market overtime, because the rising cost of widgets means a rising income for widget makers.
Ultimately the need for widgets hinges on both availability and the purchasing power of its potential customers. It only depends on prices inasmuch as the relationship between the cost of goods and the buying power of consumers:
If consumers make less money and prices stay the same, sales drop.
If consumers make more money and prices stay the same, sales rise.
If consumers make more money and prices go up, sales stay the same.
the niche that the non-slaver company occupied before was to sell more expensive goods, demand for which is less than for the cheaper salve-made goods. When slavery is banned, prices go up for everyone, and while some customers will pay the higher price, some will not. The total demand decreases. If everyone was willing to pay the higher price to begin with
You're forgetting the effect that all of those thousands (millions?) of workers who used to be slaves are now employees with paychecks and now have the capacity to buy those goods independently. Demand actually DOES increase because the pool of consumers increases dramatically. Moreover, the former slaves see an improvement in their quality of life which in turn increases their productivity; this effect is much harder to quantify, but it partially offsets the sudden rise in labor costs.
Is this the old, "If we raise the minimum wage by one cent, why not raise it by a superzillion dollars?," fallacy?
Except it's not a fallacy. It's a standard estimating technique
No it isn't. You estimate by establishing a viable upper limit and a viable lower limit and then narrow the field based on known factors. "Superzillion dollars" is an upper limit; it's easy to see that's too much, so the upper limit is too high. A million is also too high. A thousand is too high. A hundred is too high. Etc etc.
And it's a fallacy because it ultimately devolves to the strawman argument. Just because drinking 10 gallons of water in one sitting will make you sick doesn't mean water is bad for you.
What's the 4th option then??
The 4th option is that the increase in minimum wage increases the demand for products and services locally, resulting in an increased demand for workers to meet the new demand. Simply put: people have more money, they use more services, and providing more services means more employees.