That is an excellent question.
Sony's timing with PS4 Pro last year was half-way through the life cycle of PS4, exactly what you would expect from a mid-life kicker product.
If Sony has any business sense, they will have the PS5 coming out in Fall 2019, exactly 6 years after PS4 and 3 years after PS4 Pro. This strategy maintains a 6 year console life cycle. Sony is going to want to minimize the time Microsoft can claim to have the "highest performance console", so Sony is not going to wait till 2020.
Microsoft, on the other hand, has a dilemma. Scorpio is 4 years after Xbox One. If they launch Xbox Two in 2019 to keep up with Sony, the lifecycle of Scorpio is relatively short. If Microsoft waits till 2020 for Xbox Two, they give Sony a year head start.