It seems to me announcer breaks like this used to be far more common. I recall listening to games in the 70’s and '80s where the play-by-play or color for the 4th thru 6th innning were commonly called by a different announcer. I suspect that as announcer salaries grew, stations squeezed out this practice to save money.
With respect to the Cubs, I think it’s a good idea to have Cory Provus calling the game. It’s a good entry-level opportunity for people trying to break into the business. For the past few seasons Andy Mazer did pre-game and some play-by-play for the Cubs, and he has since moved on to become the play-by-play guy for the Padres.