This is false to a degree.
Rudy isn't a shooter yet, but there are dozens of examples where players who weren't shooters added one later. Malone was deadly with his mid-range game later in his career and Horford's career (and latest contract) doesn't come about as it has without him developing an outside jimmy.
Adding a shot doesn't takeaway from a player. It never has. Rather, it simply adds to their offensive arsenal and gives the defense more to worry about. Rudy adding something reliable out to 15 or better makes him so much more deadly in offensive sets where he can roll outside, sit at the top of the key, or take a shot when he's out of his normal position and the play breaks down.
Aside from that, Rudy has the unique physical attributes to remain in the league as his game relies as much on timing, anticipation, intelligence, and instincts as his athleticism. Granted, he will age, but his decline probably won't be as bad as others I think. Mutombo played until he was 42 and still had massive defensive value until he was 35.