This could go several ways, but here is my suggestion:
Rick moves to #1 for now, and probably for a while. We know him, and we're comfortable with him. He's earned the right to be #1.
Bring in a couple of other folks, possible local, to keep the other seat warm. Maybe Harold Reynolds can morph into a play-by-play guy. It's not the same as doing TV color. Not even close.
Over time, bring in someone who's been a second or third chair elsewhere and let him grow into the market and into the #1 job over a few years. Experience in the game and experience in the market make for a beloved icon. It won't happen during this decade.
Jon Miller had bounced around 2-3 gigs before getting the #1 seat in Baltimore. Only idiot managment/ownership made him move to San Francisco. He'd turned down the Yankees and other big bucks offers to stay with the O's.
Michael Kay went from Yankees beat writer to radio color guy, waiting a couple of years before they let him do an inning or two of play by play. A decade later, he was splitting PBP time with John Sterling, then becomming the primary TV guy when the Yanks formed the YES Network.
As for the pull of baseball, consider this: Bill Schonley had already acheived 'beloved' status in Portland by the late 70's, but pushed hard for the Mariners job. He had been the voice of the Pilots and many other teams around Seattle in the 60's. How 'interesting' would it have been to call the '77 NBA championship, then head up the freeway to call the Mariners?
And, dare I say it: Dave was very good, but not great, when he came to Seattle, graduating from the #3 chair with the Angels. He grew into the gig and transcended the comings and goings of the team: players, managers, owners and venues to achieve much-deserved Hall of Fame status.
We'll get one of those 'signature' voices again if management realizes that it will take a decade. And it will happen with the right guy - someone who (like Kevin and Dave and Rick) comes from somewhere else but decides to become a Seattle lifer.