As it relates to the OP, it matters since the discussion is whether or not the Mosaic covenant is the OT administration of the one CoG and whether or not Hebrews 9:15 provides a definitive resolution in favor of the proposition.Maybe we are having a terminology issue. I am not sure how.
I mean, I see now that the "CofW ending" refers to the means by which any one born human could achieve eternal life has ended, ok. Agreed. And so when I said the CofW condemns still today, you all agree with that. I think we are on the same page there now.
But does it even matter that you were referring to the Mosaic covenant vs. the CofW?
As I understand, in the days before the NT, each covenant has one (or more) element(s) of CofW: "do this and live" complimented with element(s) of types of Christ in which grace (by faith alone in Christ alone) saves those called as true Israel and OT saints.
I said above that I don't think the text does that itself, but is another leg in the argument bolstering the proleptic nature of saving grace in the OT present within the OC. If saving grace is part of the OC, If it's institutions, forms, sacraments, were intended to present Christ as the object of faith through types and shadows to the elect OT believer, then it is indeed an administration of the CoG. It can be no other unless one wants to posit a hybrid of CoG and CoW, which is nonsensical, or to deny the salvific component of the OC, which I think is untenable. If the priestly sacrificial system was intended to present Christ to the people of God in types and shadows and if the law's "do this and live" was intended to point people to the need for what was presented in the types and shadows, it is an administration of the CoG. A covenantal structure that presents and administers the promise of God of the righteousness that is by faith.
Now, I know my Baptist brothers would certainly take issue with things in the above. I'm not trying to convince or fully demonstrate. Just better presenting as clearly as I can my thought process for your clearer understanding.