No one plays chess purely based on theoretical knowledge. Chess is an intuitive game. We can sense "good" and "bad" positions. Why would a player in the 19th century have trouble with this? We may improve due to new ideas, but those new ideas come from innovative players.
Steinitz lived in an Era of attacking chess, yet he placed emphais on defense.
The "spirit of the age" argument is faulty. If that was the case, chess would remain the same.
Someone eventually has to think outside the box.
Pretend I photoshopped a chess piece in
That's me :D