You and I agree precisely on the topic, esp. on the batshit-senile period being fun but not necessarily good reading.
But c'mon 'Unsavoury Dude' would just find something in Kornbluth or Niven to justify his underage banging needs.
The excuse is not the problem.
Well, it facilitates a corollary problem. Essentially, science-fiction fandom is seen to varying degrees as a refuge from social exclusion by many fans, making them gunshy about acting "exclusionary" of their fellow fans in turn. When somebody can justify behavior that wholly warrants exclusion, banishment, and possibly a restraining order in terms of an author whose writings are one of the core pillars of the 'traditional' science-fiction community, it makes it very hard to get anything done about him even when everyone acknowledges that his behavior is a problem. There is nothing in Niven or Asimov that could be so readily used to justify this behavior, and no other author has the kind of weight of fandom force that Niven, Asimov, and Heinlein do. (I may be leaving someone off the list as a momentary absence of mind.) Even other Great Names do not carry the kind of invokable weight that these three do, and Heinlein's views on consensual morality are invokable for some pretty undesireable behavior.
So, to summarize: I don't think
Aaron Unsavoury Dude would find the same justification in Niven, and if he found it in Kornbluth it wouldn't give his "moral stance" the same kind of social shielding Heinlein does.The Heinlein Problem is not so much in Mr. Unsavoury's behavior, but in the behavior it creates (or inhibits) in those around him.