They want to annihilate the second hand market, so every copy sold is through them. Trading in your old games you don't play to buy the next one is a staple of console life for many, but Sony doesn't get a cut of the second hand games sold.

CeX is the place to go to get games near me, seeing as Game got swallowed by Sports Direct and hollowed out leaving Smyths, Argos and supermarkets the place to buy new games. Sony/Nintendo/Microsoft would love CeX to implode but it won't happen whilst there are a steady stream of console gamers trading in the latest games to pick up an older one they never played or a random movie.

They've publicly declared to their investors that they can't expect a growth market anymore (perhaps because of RAMpocalypse, or maybe consoles were just tapped out anyway), so they're instead focusing on maximizing revenue per user.

They're transitioning the facility used for disc pressing into making microlenses for their optical sensor division (which is the most profitable part of Sony)

Keep in mind that Sony can also be betting that not everyone will stop buying completely. Just because 35% bought this game physical does not necessarily mean that when they stop selling discs, they lose 35% of sales. Only half, or a third, or a quarter, might stop, and Sony might be betting that most will just suck it up and continue buying.