There's no argument that milf is subjective. However, it's not something that actively interrupts the searches done on the website, the tag milf by itself is being used otherwise correctly for the one thing it describes, even if it goes against all the rules of form. It's existence, as obnoxious as it is, works just fine.
Thus, leaving it as it is lets the function of searching for those items continue unhindered. But beyond that, replacing milf with mature and having milf redirect to that is just good practice and sets the problem down for good.
Removing milf without a suitable replacement will cause problems for searches of those images in the future, getting in the way of making those images with the tag as it is, no longer directly searchable. Even if mature has negative connotations, which I agree make it a kind of loaded term, it is form and function-wise a superior option, and looks like it has earned the majority of support from past users.
That all being said, since Apollyon and NWF Renim suggested a pool instead, I'd say "MILFS" or "MILFS Only" is an OK name, and give it the agreed criteria: "Older women that can be considered attractive or who've retained their beauty, a woman doesn't have to be a mother to be considered one. This replaces milf tag."
My personal feeling is also to consider what Schrobby said, that mature is a better tag for general things like cougars and non-milf mature characters, then have the actual milf images be in a pool, probably the most preferable idea all around.