At the start of the season Fonte looked very shaky and was underperforming hugely - the difference between then and now is remarkable and to have the ability to step his game up as he has done more than earns him the right of a regular spot. After all the criticism and doubt the back 4 received it is a credit that they have managed to improve and show they are able to cope in this league. The Fonte-Yoshida partnership has proved to be effective and there's no reason why it can't continue to be that way.
That being said, I agree that Yoshida is the weaker of the 2 and probably was in part a 'panic buy'. With match experience, I can see Forren and Fonte developing as a very capable pair.