The capable nobles may not necessarily be loyal to Carlo. After all, these nobles are local factions of Spain, having been rooted in Spain for at least a hundred years. They certainly won't be overwhelmed just because Carlo becomes the King of Spain.
This is also the reason why Carlo extensively employs new nobility, constantly granting new noble titles. Compared to these old-fashioned nobles, Carlo trusts the loyalty and capability of the new nobility more.
As for these old-fashioned Spanish nobles with complex powers, if they do not hinder Spain's progress, Carlo naturally does not wish to interfere.
But if someone creates trouble on Spain's path forward, Carlo will let them know how supreme the king's power is over the nobility.
Since the king has the power to confer noble titles, he naturally should also have the power to revoke them. Although Carlo has not often used the power to revoke titles, this does not mean he lacks this power.
