Federer definitely pioneered the game. No doubt - but the game caught up to him. The only guy who had a forehand comparable was Nadal and Fernando Gonzalez, and that's all he had.
Del Potro forehand is another level, now you have huge hitters like Thiem and even Kyrgios hits and enormous ball. Everyone hits as big as Federer now. He didn't get worse, others caught up.
What's so difficult to understand about people have caught up to him? If he didn't improve he would have became Hewitt, who was stagnant and if you're not getting better, you're getting worse.