If there is any, there is not much separating Carvalho and Samuel in terms of quality, so I don't think an exchange is worthwhile now. Samuel has become content with the rotation system and fights for his place in an honest way. All in all, there is nothing negative to be said about Samuel right now, especially not since he has hit this form. Why exchange a player that suits the team very well with a player who is not a direct upgrade and perhaps won't gel as well as the current player?
It's like exchanging Julio Cesar with Frey. Not really any reasons for it.