From a recent interview with Sony Interactive Entertainment Vice President Hideaki Nishino:
"In terms of compatibility, it is important to move PS4 titles on PS5, but I have insisted that forward compatibility (supplying the same titles as PS5 to PS4) is important," Nishino said.
"The current assumption is that the transition from PS4 to PS5 will take about three years. In the meantime, how can I keep buying games on PS4? Can the purchased games be played on PS5? That is important."
"At a certain time, we ask developers to develop on the premise of "cross generation" of PS4 and PS5. Of course, we also need to improve the library to make it easier."