Not saying your wrong, cause everything you said makes sense, but i've heard from others on the forum that after a while, all that clutch material in the old fluid is the only thing keeping it going.
So no complete flush, but maybe a pan drop and filter and kit would be a plan. Either way if your trans is slipping now, the only true fix is a new or rebuilt trans.
I would agree within reason, except his fluid is burnt. Any friction material suspended in the fluid is torched now and not doing anything anymore. If it were still somewhat red just had stuff floating in it, I could see trying to limp it on.
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