Should never be forced to make something that doesn't fit with their style or causes them not to be able to turn a profit. The number of people with money outside of what is considered a fashionable physique means there is huge profit to be made from catering to an underserved group of fashion conscious shoppers. Someone is going to realize this and they will make a lot of money being the first big designer to have an all inclusive brand.
Big-name fashion brands should not be forced to cater to any client that they don't plan to. Fashion is a creative field, and it seems irrational and misguided for people to sit on the sidelines and give advice to professionals in such a field. Big-name brands should, from a business standpoint, sell clothing to people of all sizes, simply to increase profits; if they're not doing so, people must respect their decision.
Fashion is not a necessity, though it may feel sometimes like it is. Fashion is a luxury. Social institutions have a moral duty to feed the hungry and see that people have shelter from the storm, but need not see to it that everyone has a Coach wristlet, a Birkin Bag or some semi-sheer hundred dollar LuluLemon yoga pants. People of any size, age or appearance can buy sweat pants
to clothe themselves, and those who keep themselves slender and fit need not feel bad about it. Nobody has a right to fashion.