To be clear, and I said this at the start, Star Trek V does have some good scenes in it. It fails overall, but, in isolation, there are some good bits.
Wrong to direct? No. Even though I believed he failed the exam, I also believe he got somethings right. In retrospect, I do think his desire to direct was more out ego rather than the more noble ambition of telling a good story well. For Nimoy it was about the work and the art. For Shatner it's about the more superficial aspects. You know what, he's not the only director to be that way, and some of them are tremendously successful. I
I did like the "our pain is part of what makes and defines us." I like it because it was a logical offshoot of episodes such as "The Enemy Within" as it speaks to larger, though unintended point, that a perfect world would not guarantee a race of perfect people. Probably just the opposite.
I agree that Star Trek V just needed one more top-to-bottom rewrite (I've said it myself in the past). A rewrite that would have heeded what Harve Bennett, and others, tried to communicate to Mr. Shatner. He didn't get more help because he wasn't being receptive to it and that put others in the position of not wanting to seem obstructionist. Honestly, I also do believe that Paramount had little to no faith in his ability and from the beginning probably worked to cut their envisioned losses and move on. A consequence of that is Shatner was, in a sense, setup to fail. IOW, Star Trek V's failure was the result of a self-fulfilling prophecy.
As for the B.O. competition, I don't buy it. There is always competition. According to B.O. Mojo Star Trek II was up against E.T., Rocky III, and Poltergeist. That year it ranked as the 6th most successful film of the year (1982). I mean read the list of films released in the month of June 1982 and WOW!
As opposed to Star Trek V. It premiered in the number one spot (beating out Indy 3) then quickly sunk like a stone against, arguably weaker competition. Indy 3 Ghostbusters 2, Karate Kid III all did well but deemed much weaker than their predecessors. Star Trek V finished the year in the 25th spot. (1989)
So, again, the competition argument doesn't hold up as an excuse for it's box office failure.
It was a massive blockbuster summer.
Last Crusade (certainly not weaker than Temple of Doom BTW)
Ghostbusters 2
Karate Kid 3
Dead Poet's Society
Lethal Weapon 2
Star Trek V
Honey I Shrunk the Kids
And the MASSIVE blockbuster, Batman
While I think word-of-mouth and critical reviews certainly contributed, absolutely don't agree that "box office competition is just an excuse" either. TFF had considerable competition that summer.
Some consider summer of 1989 to be one of the best movie seasons ever.
https://nypost.com/2014/07/07/was-1989-the-best-summer-for-movies-ever/