So to make your point you have to make the distribution of customers even more unrealistically clustered, when in reality it will almost always be a smooth curve. And the need for this restriction also proves that price discrimination is not always good for "everyone," since in the original case it clearly isn't good for consumers.
But you're also ignoring the point about competition. If the goal is to maximize overall (combined consumer and producer) surplus then price discrimination at best works only as well as competition would if it existed. In a competitive environment a theater can't charge $6 to people who would pay $6 and $10 to people who would pay $10 because the theater across the street is charging $5 to everyone. And that creates more overall surplus because there will still be some students who would pay $5 but not $6 and some adults who would pay at least $5 but not $10.
If you're still not getting it we can go for the reducto ad absurdum. Suppose that food sellers collectively come up with a way to engage in perfect price discrimination and all begin charging that amount. Since without food people die, that amount will always be every last dime the buyer has. I can go into how and why that would be bad if you really want me to but I think it should be pretty obvious.
> Moreover your analysis is wrong - not everyone is as happy as they could be, you're only thinking from the consumer's perspective.
My entire point is that it isn't possible for everyone to be as happy as they could be in a purely distributive scenario. If there is $3 of surplus on the table then whoever doesn't get all of it is going to be less happy than if they did. And all price discrimination does is make sellers happier at the expense of consumers. It has no effect or, if done less than 100% perfectly accurately, a negative effect on overall happiness.
> It is not discrimination in the political sense, but the dictionary sense: to differentiate.
I don't think anybody is confusing it with e.g. racial discrimination. Although I suppose it could create a statistically significant difference in the prices charged to different races, which doesn't exactly help your cause.
> Price discrimination in theory should not drive customers anywhere but rather make them indifferent to their choice of supplier.
If one supplier is engaged in price discrimination then any other will have the opportunity to take their business by charging a lower but still profitable price to the customers being charged the higher price.