At best, your argument can be used to support invocations of executive privilege to those made by the sitting President, not to only those made on behalf of a sitting President. The reasoning for applying executive privilege to discussions with the current President, that ey can receive unvarnished advice from them subordinates, applies just as much to ex-Presidents. As I expressed earlier in a different thread, I expect that SCOTUS will first decide whether the information currently being requested meets the requirements for invoking executive privilege on behalf of a current President by the current President, and only if it does (which I expect they won't, tho I may be wrong) then go on to explore the issues of whether it applies on behalf of an ex-President and who must sign off on it.
Yes, I am arguing that there is no right of executive privilege granted to ex-Presidents apart from that granted to the sitting President. The Constitution is clear as to how the executive power is granted. A limited privilege for an ex-President can only exist so long as the sitting President is willing to exercise it on behalf of the former President. If the sitting President is not willing to exercise executive privilege on behalf of an ex-President, that should be it.
Why? Executive privilege is not an enumerated power of the executive. To the degree it has been inferred as an executive power, the reasoning has been that executive power requires being able to receive confidential advice on what to do when in office
from his subordinate officials, so long as that advice concerns the lawful exercise of his executive powers. If that confidentiality becomes subject to the discretion of a successor, who may well be a political adversary, it is far less secure and effective and thus weakens the value of executive privilege. I realize that our constitution is extremely naive when it comes to politics, and for that reason, I don't consider your argument entirely wrong, but since the whole scope of executive privilege is one of those constitutional penumbras inferred from the text, I find it runs counter to the whole rationale for having executive privilege in the first place.
However, since the information being sought neither concerns the lawful exercise of executive powers nor advice from a subordinate official, I agree that executive privilege doesn't apply in this case. That would be so, even were Trump still President. Hence, there is no need at this time for SCOTUS to decide if ex-Presidents can invoke executive privilege.
In any case, it would be wise to keep in mind that any decision concerning the invocation of executive privilege by ex-Presidents will not affect only Trump, but all ex-Presidents, including Democratic ones, if a future Republican Congress seeks information actually covered by executive privilege but a future Republican President declines to invoke executive privilege.