I'm not sure I understand the usage of the term "legacy" here. To my mind "legacy" refers to an old, "grandfathered" product, possibly one that's closed to new applicants. Citi's own (non-co-branded) cards are not "legacy" in that sense. Some, like the DC, have been around for years, but they are still active products. And Citi just recently launched the non-co-branded Custom Cash, demonstrating that it continues to be active in developing its non-co-branded card offerings.
In any case, the overall consensus is that the co-branded cards are somewhat easier to get - same with any other issuer, from AmEx to Elan. Keep in mind that the fact that the co-branded cards are somewhat easier to get doesn't mean they're somehow "worse" or "lesser," especially in the case of the Citi Costco card, which in terms of cash back and ancillary benefits (like extended warranty, rental car coverage, no FTF, etc.) is one of Citi's strongest offerings overall.
