I once asked a professor to marry me - more precisely, to perform a marriage because he was a minister of the Universal Life Church.
He used a similar retirement argument, pointing to a younger professor who had similar credentials. I referred to his seniority and acclaim as the reason I'd consider only him, he found it sufficiently amusing and/or complementary, and said okay.
I will retire in 2 years.
I would first congratulate well, then say "I won't bother you further, but because of your X, Y and Z I think that even a short time interacting with you would be an incredible experience for me, and this way you won't have to bother with all the paperwork and aggravation of the graduation part."
It's just possible the first "No" is not the final, and may just be a check to see just how serious you are and how willing you might be to change primaries later.
However you may want to do a quick check to make sure they currently have students. If it looks like they've emptied their nest already, or nearly so, then maybe it doesn't make sense to ask again.