His lack of power is his weakness these days. Not his serve, return or any of his tennis skills. He's just a bit weak these days compared to the strongest players.
He had to slice just to stay in rallies many times vs Berdych. Not gonna work if the opponent doesn't gift UEs
The lack of power is most evident on his backhand side. His forehand can still pack a punch
It's a consequence of getting older.
And with the current type of tennis that demand power and stamina, it's getting more and more difficult for him.
Had it been the 80s like in Connors era, Fed would have still contended strongly for any titles.