I doubt he's joining to Real Madrid as a bench warmer. Casemiro is still in his prime & he won't slow down anytime soon.
Casemiro will be 29 next year and he doesn't have a backup. Madrid has also shown they don't hold into their starters for as long as we are once they start to decline/underperform.
Zidane loves Camavinga and imo he could also play as a box to box midfielder (so in Valverde's role) which would guarantee him more minutes. Many young players over the recent years have choosen Madrid because of Zizou and even if some of them like Ceballos or Brahim never got a fair chance this won't prevent other youngsters from wanting to play for Zidane/RM.
They could also buy Camavinga and loan him back to Rennes or even a club like Dortmund for a year or two to develop further before they instert him into their lineup. They can afford that, we can't.
So I think he's still Madrid bound at this point.