He has to be replaced as he is getting old & the workload is too high.
Best in that position ? I'd say probably yes. I can't think of anyone who could currently do the job(s) Pep asks Fernandinho to do, better than he does it. One of the best centre mids I've ever seen certainly. And with a little help, rather than being put next to Yaya or a collection of attacking mids who keep giving the ball away in their own half, would have no problem seeing off Liverpool imo, which may be what happens next season with him & maybe Jorginho.