Trying an extreme form of an argument about tipping to see if it works:
Generally speaking, I don't consider myself to have an obligation to undo structural injustices. I have a responsibility to act ethically in my own interactions with others, and I have a responsibility to work through representative and democratic means to change unjust situations (and, let's be honest: I will choose some subset of unjust situations to devote the most time and energy to, no offense to the rest). But, barring a severe injustice whose very existence threatens the legitimacy of a society, I have no particular obligation to do more than this.
In the United States, we have a set of laws and norms in which we have chosen to a. allow some workers in the food service industry to receive less than minimum wage and b. accepted that a tip is to some extent a reflection of the quality of service received. It appears relevant that for a. not every food service worker receives less than minimum wage, and a customer has no way of knowing this, nor exactly how much less, nor the pattern of accumulation of tips on a given evening, nor how the tips are distributed among the waitstaff (and possibly others).
Given the difficult of changing a., the emphasis is now being put on b., but in a form that is a nonsensical mixing of individual and collective social responsibilities: i.e. each person should act as though their tipping would constitute the norm, but should also tip as though other people will neglect the norm, and the responsibility of making up for a shortcoming in income falls solely on the tipper. But, to highlight one instance of this, neither unthinkable nor unobserved back when I lived in the wilds of Michigan, one can imagine an evening where one comes to a restaurant, but one's server will have only five or six customers--i.e. almost certainly lose money on the evening. Given the structural realities of the sub-minimum wage and the necessity of splitting whatever tips there might be, it seems the general logic of the changing tipping norm would dictate that the customer should tip more in that scenario, even with all other things being constant. Except that this is an absurd result.
(An aside, relevant for the morality and etiquette questions involved: a number of articles that emphasize the importance of tipping generously emphasize that the tip should be in cash whenever possible. It's hard to think of a justification for this that is not an attempt to provide cover for unjust or unfair action farther down the line: lying about tips, withholding from the common share of money, etc. The structural injustices of the wage system cannot possibly justify both increased ethical concern on the part of the customer and decreased ethical concern on the part of the waitstaff.)
Part of attempting to work this out requires realizing that waitstaff constitute a group with interests that may be orthogonal to justice, fairness, or common sense, no less than restaurant owners who see the logic in paying as little as possible to their employees. They're free to make whatever demands they want (everybody needs to tip 50% all the time!), and we're free to ignore them. It can't be the average person's job to remedy a structural injustice every time they want lunch, because 1. once you start playing the 'remedy a social injustice' game, it's hard to do anything else and 2. waitstaff should, on that logic, lose out on those marginal dollars in favor of people who are more deserving, of which there is (probably) no shortage. And, it has to be said, though it's often ignored in threads about tipping, a generally-enacted policy of "well then, don't go to a restaurant where you'll have to tip" is the worst possible outcome for the people involved.
None of this allows the individual customer to simply neglect their responsibilities under b. to provide a tip that reflects the quality of service received, but it also means that the inexorable march to higher tip percentages need not be accepted.