This has nothing to do with casting. They can recast Kirk, Spock, Uhura, Chapel, M'Benga, etc. But I would REALLY like to see less reliance on legacy characters. TOS Kirk went dozens of episodes without running into another starship or meeting another captain. What I really appreciated about the original series was that starships were really spread out so they couldn't get assistance quickly. Bob Justman resisted Kirk having a regular Klingon adversary because of the vastness of space. I bristle at Small Universe Syndrome. Having Kirk do a one-off in an alternate future? Awesome. Making him recurring? I wish they wouldn't. I'd rather they leave out all of the original series people other than Spock, until we get a lot closer to Kirk taking command. It just feels like they're spending more time on Spock, Chapel, T'Pring, M'Benga and Uhura than on La'an, Ortegas and Una. The one new person, other than Pike, who was all that fleshed out wasn't even a legit regular (miss you Hemmer!)
If I had one damned wish, it would be to spend more time on the original people, Pike and Number One.