The biggest downside might be Peck's availability and price. Usually cast expenses increase overtime and he'll be the longest serving actor. Will he even want to continue?
It's a new series, that means an entirely new contract and salary negotiations. He'll have negotiation leverage, but so will all the other TOS actors. And they all know that if they ask too much they will sink the show and not get paid at all.
Disney was notorious for cancelling shows and relaunching spinoffs with the same actors to keep salaries down.