Oh, I know you're not trying to be direspectful to a dead man and I see where you're coming from. I actually agree that TPTB thought so, about both David and Saavik, especially when you consider that both characters were originally added to inject 'youth' into the crew, and, in some ways, supersede their older counterparts so the franchise could continue. Earlier drafts for the script of TWoK place much greater emphasis for this and even made David Marcus a sort of 'rebel leader' on a planet taken over by Khan. From what I understand, though, Harve Bennett also felt films had to have a 'karmic' balance - the death of Spock was in exchange for the salvation of the Enterprise, the death of David and the
Enterprise in exchange for the salvation of Spock. I agree that clearly no one was that fond of Robin Curtis, and even I always preferred Kirstie Alley, although perhaps it was more the character approach - half Romulan vs. full Vulcan.
I imagine Shatner might have felt threatened by David in a similar way that Kate Mulgrew felt her job a bit endangered by Jeri Ryan, but obviously it didn't work out the same way.
There's actually a somewhat relevant quote (the only one) from Merritt Butrick's imdb page:
I'm part of a legend. I gave what I had to give at the right time and place for my own personal gratification. I look at my resume at this point and it reminds me of how well I've done in the few years I've done it. What I'm working on in my career is longevity and not instant success. I haven't particularly found the leading man image that I have yet to give, but I certainly explored certain aspects of him. Hopefully, I'll look in the mirror someday and he'll be there.
Poor guy.