I’ll give my points as to why even a good vinnana is contaminated and why an Arahant can’t have any sobhana cetasika in another reply later.
But before that, I want to highlight the sutta Siebe mentioned(my thanks to Siebe as well for sharing this sutta.)
AN 3.34:
“(1) “Any kamma, bhikkhus, fashioned through non-greed, born of non-greed, caused by non-greed, originated by non-greed, is abandoned when greed has vanished; it is cut off at the root, made like a palm stump, obliterated so that it is no more subject to future arising.
(2) “Any kamma fashioned through non-hatred, born of non-hatred, caused by non-hatred, originated by non-hatred, is abandoned when hatred has vanished; it is cut off at the root, made like a palm stump, obliterated so that it is no more subject to future arising.
(3) “Any kamma fashioned through non-delusion, born of non-delusion, caused by non-delusion, originated by non-delusion, is abandoned when delusion has vanished; it is cut off at the root, made like a palm stump, obliterated so that it is no more subject to future arising.”
^ That matches what you had written before the revision:
“One actually strives to remove greed, hate, and ignorance, which are “san”. When this is done, other three causes are automatically removed. ”