I would say yes, he is past his prime but he still is very very close to it. And even though im saying he is just past his prime, I still think he is the most complete driver of the grid due to his unmatched consistency.
Now, why I say he might be slightly past his prime is his qualifying performance in the 2019 season. Lewis has the same amount of pole positions as Bottas and while he still convincingly beat Bottas in the overall quali battle, it is rather surprising that he did not have more pole positions when the car was capable.
But we also have to give big credit to Mercedes obviously, they are creating that dominant car, they are allowing Lewis to win the championship, it is not just the best driver winning, it is also the best team winning.
Max might just be slightly faster in raw pace but Max's driving style is still very immature, he always takes the most aggressive action and unless he has a dominant car and a weaker teammate, I do not see him winning Championships. But history says, the dominant car always wins anyway, so you can kind of question if the driver even matters that much. I mean tell me the last time, somebody not in a dominant car won a championship. We can only be hope that there might not be one dominant car in a season.