Next gen PS5/XB1SX sales ratio will look the same as this gen's. Huge changes in console marketshare require huge changes in the industry.

[UnderwaterFunktown]

I don't think the ratio will be quite the same, because as you point out yourself Xbox screwed up majorly this gen and I think they've learned their lesson this time.
That being said I still I think the change will be minor, because Playstation is just a stronger brand everywhere in the world except possibly the US.
Currently the split is around 70-30. My honest guess for next gen is something like 62.5-37.5