Reality: No store is so small as to prohibit such a division. There's literally nothing to stop a store from diving up clothes by age or size to any arbitrary degree of precision that doesn't imply empty categories (there's probably no clothing at all that belongs in the '8 years, 3 months and 2 days' section).
Have you ever been in a store that sells clothing?!?!
Look at how it's organized--each rack generally contains one or a closely related set of garments in various sizes. Rarely do you see unrelated garments on the same rack other than in clearance situations. Rarely do you see multiple racks containing the same garment. If you try to divide a group into two you'll end up with half-racks of clothing, you'll take up a lot more floor space to sell the same number of goods. If you try to combine disparate garments on the racks so as not to waste the space you'll end up making it a lot harder for the customer to find what they want because you only get a good look at the garment in front--it should be representative of what's behind.
There's opposition to fucking everything, including not changing anything. The existence of opposition isn't a reason not to do something.
Reality: Prefacing your unsupported claims with "Reality:" adds exactly nothing to their believability.
Why are women going to want to go through the hassle of switching? Especially since most women's garments are cut differently than men's and not interchangeable even if they look the same.