And now for some generational comparisons...
7th gen gap for PS3/360 was around 3.4m with 360 20th month in June 07 and PS3 June 08
8th gen gap for PS4/XBO was around 11m with both of their 20 month marks in June 15
9th gen gap for PS5/Series is around 5.5m with both of their 20 month marks in June 22
Currently the 9th gen is half what the 8th gen was for the same time.