For their own stuff, Mozilla has talked in the past about specifically just only trusting intermediates they've seen. Their modern root agreement says the CA will disclose all relevant intermediates, so arguably they have a complete list and further if that CA "forgot" about a few of them whether that's because your firm is too disorganised ("Oh, right, the one on Barry's laptop, forgot that one") or an actual attempt to sidestep this rule - then we certainly don't want to trust these undisclosed intermediates, do we ? They're either purposefully shady, or they're forgotten about and likely never have been audited.