I think they're afraid of that becoming the Ben Affleck Jack Ryan.
You lost me. Unless you mean that Renner would be the George Lazenby Bourne. Then I get it.
I suppose they want this to be like the TOKYO DRIFT of the BOURNE series and have Damon come back later, but I don't think that approach will work with this series. They should just wait until Damon is ready or reboot it.