I don't think it can be as late as 2021 for two factors
1) a year or more behind MS, highly unlikely imo - would give MS a gigantic lead even if they haven't made the most of the hardware.
2) While its a brand new cpu / chipset now, in 18 months -2 years it will be pretty old hat and Sony wont be anywhere near bleeding edge for the hardware even considering its a console.
I really wouldn't be surprised at Easter 2020, but maybe Christmas if they absolutely have to (but imo that will give MS a good start in the sales wars).
Back compatibility is no longer an issue as chipset will be practically identical to ps4 (unlike other ps models generation to generation) they just have to get BIOS working correctly so hardware throttles to the right speed etc.