PSA should not start subgrading. Uniformity in the label is what a lot of collectors like about PSA. If PSA introduced subgrades, will cards with the same overall grade but different subgrades be valued differently? There’s so many new grades and nuances that will be introduced if subgrades are involved.
Also we already have the old vs new cert argument, adding anything else to fuel the “weak vs strong X grade” argument is meh.