You are so right! I do love how we (I) get in a lather about hypotheticals. LOL. But I like your persistence :).
I don't really think his age is all that germane to the plot. I mean, when Groff took over he kind of made it a sexy king thing more than, I suspect, BDJ. (This is all incredibly subjective, of course. Groff just happens to do it for me, as do Rannells and O'Malley.) An old king for me would deliver less sex. Of course, gerontophiles would feel differently.
I hope that I have expressed that I would never consider anyone's casting "wrong". Only for me, some casting requires some deep breaths and consideration, never a bad thing. Someday I'd be curious to see a production of Hamilton with some White folx in the mix and see if a show conceived for BIPOC actors could work with non BIPOC's. (Not holding my breath that that would happen any time soon.)