"Kodera also noted that while Sony is investing in first-party, it is not entirely for the creation of new IP. The refreshment of existing IP and development of successful IP as franchises are also part of the company’s plan to strengthen content."
A.K.A Horizon 2 is coming. There's been basically zero sequels to exclusive PS4 games so far, as they start to move developers onto projects for the PS5's early years they'll probably begin to make games that make use of the existing assets and engines from previous games to continue to release software on PS4 with less work.
After all PS3 had 3 Uncharteds, 3 Resistance's, 4 Ratchet's, 2 Infamous Games 2 LBPS, 3 Motorstorms, etc. This generation so far has had next to no first party series with multiple entries. So they'll want to start cranking the machine to push out games that can be made with less work due to the foundations already being in place, freeing up staff to start more long-term projects for the PS5.