I disagree. Turner has effectively guarded the 1-3 at different times during the season. There have been times when Stotts specifically put him on the other team's PG. I don't recall Stotts ever inserting Napier into the line up when we needed a defensive stop.
I do not share your optimism about Crabbe ever becoming even average as a defender. He just doesn't have the mindset or the motor. Watch how Turner gets up to guard LeBron. I can't see Crabbe ever getting excited to take on that kind of defensive challenge.
I also don't understand why you think Crabbe can improve defensively, but Turner can't improve his shot. There are plenty of examples of players who greatly improved their 3-point shooting in their late 20s or early 30s. Here's just a few off the top of my head: Magic Johnson, Kevin Johnson, Jason Kidd, Rip Hamilton, John Stockton, Ricky Pierce, Mark Jackson, Boris Diaw, Cliff Robinson, Channing Frye, etc. I'm sure there are a lot more I can't think of. Those a few Hall of Famers, a few ex-Blazers and and anyone else that randomly popped into my head.
I think I'd be hard pressed to come up with a similar list of guys who greatly improved their defense later in their careers. I'm sure there are several, but most of the ones I can think of are big men who learned how to defend without fouling.
In any case, I don't think Turner needs to become an elite 3-point shooter to be more valuable to the team than Crabbe. I think if he can improve similar to what Aminu and Harkless did the last two years, it would make him a more valuable player, and others can pick up the 3-point shooting slack (Napier, Connaughton and anyone we happen to draft) for losing Crabbe.
BNM