Your conclusions are nonsense.
Getting regular work as an actor for over 40 years is proof of success. He appeared in some 326 episodes of "Law & Order: Special Victims Unit," one of the longest lasting shows ever. He was good enough to keep the gig.
You may not like his acting, and that's fine. But he has nothing to apologize for.
Being an actor who plays a particular role involving a specific profession or character doesn't make that actor capable of holding that position in real life. It doesn't make Melissa Benoist a super hero even though she can pretend to be on in "Supergirl."
Let's get real, shall we?
Nobody says you should take Belzer's books seriously because he's an actor. Would you have taken the book on crop circles, "Wild Things They Don't Tell Us," from the late rock singer Reg Presley seriously because he found fame and fortune singing "Wild Thing" as a member of the Troggs?
Sorry, man, it doesn't compute.
In saying that, I think Belzer would be a fun guest regardless of whether you can take his theories, borrowed or otherwise, seriously or not.