Agreed...do you think he would have recast any of them since at that time none of them were big stars...
Not unless he was planning to show the characters at much younger ages. I think the whole point was, if the show wasn't going to have a lengthy enough life on TV, Roddenberry still wanted to make use of the world-building he'd already done. Casting wasn't a consideration at that embryonic stage. As people have said, it was more of a throwaway statement at public speaking engagements, something to gauge audience support.
By about 1977, though, recasting
everyone was in his mind, as he played with ideas for a movie (that became, at various points, a telemovie, "ST Phase II" and ST:TMP), because he surprised Walter Koenig by asking him if he'd be interested in playing Checkov's
father. Paul Newman and Robert Redford's public success in "The Sting" (1973) was fresh in everyone's minds and they were put forward as a possible Spock & Kirk team for a blockbuster ST film.