Why would we count Casemiro considering he's not a centre back. He's someone you'd play there in a unprecedented injury crisis, every team technically players who could play out of position in a horrendous injury crisis, that doesn't count as depth. And if he ends up at CB it means you've lost something in midfield, the same way if Shaw is forced to CB we've lost something at left back.
If you're doubling up on players covering a 2nd position then you're losing overall depth.
If Dalot could play CB would you consider going into the season with AWB, Dalot, Varane, Martinez, Shaw and Malacia? That would still leave us 2 RB options, 4 CB options, and 2 LB options, but only 6 defenders in total compared to the 8 we have now so it wouldn't make sense. It's never a good idea to cut the amount of options you have just because a player can cover multiple positions if needed, particularly when you have injury prone players. 2 first team players per position is standard at this level, I see no reason for us to voluntarily go from 8 defenders to 7 defenders when we're trying to improve. If we don't have the money to replace Maguire we need to bring in someone cheap, not just forget about it and hope for the best.