Recast. Nothing about Pine is irreplaceable, he brings nothing special.
I enjoyed all three Trek movies but it's true what you wrote. Diminishing domestic box office and the foreign box office for the last film could not help turn the tide. But Paramount does not have too many franchises so I guess it's a case of having little to nothing and trying to salvage a sinking ship.
If they can't get Pine back just re-cast the whole film. It would just seem awkward to have a new Kirk with the regular cast. While Anton did a good Chekov the character in TOS wasn't part of the cast until the second season and his character could always be explained away by being transferred to another ship and Jaylah could be the new navigator.
I don't know, there seems to be various struggles with the Star Trek franchise for various reasons.
I'm honestly not sure they should re-cast or not. Everything just seems to lack direction regarding the franchise.
Despite some production shake ups Discovery seems to be doing pretty well, and they seem to have confirmed some sort of TNG revival with Patrick Stewart, so the franchise as a whole seems to be doing better. However, I think the TV and film ownership of Trek is a bit split, and this caused Abrams some problems when marketing the last few movies.
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I don't think they even need to recast him. Just to a Star Trek movie without Kirk.
Star Trek doesn't HAVE to be about Kirk. The Star Trek franchise is a big enough draw without that character or without the actor Pine. Say he was promoted to admiral, give Spock the Captain Chair, and you're fine.
Shatner: "I can still do it, dammit. We just might have to dye my chest hair."