So I collected some information. This past record should give us some insights into how PS5 and Xbox Series consoles are stacking up against each other and may give us some idea on what the sales ratio could be by the end of this generation.
Xbox One sold 10 million units by Nov. 2014. PS4 had sold ~14 million units in the same time frame. Ratio of 1.4, pretty much the same as it is right now.
Source.
The sentiment at that time was that "Sony's winning streak might be coming to an end soon."
In April 2015,
Xbox One outpaced PS4 (deja vu!), and Xbox One sales were reportedly up by 63% from last year.
The sentiment at that time was that, because of the price cut and other reasons, PS4 will have a hard time competing with and outselling Xbox.
Then from late 2015 onwards,
Xbox One sales started declining.
I believe that the current-gen is following the same trends as the last-gen did. Early adopters purchased PS and Xbox when both systems launched amid supply shortage. PS4 outsold Xbox by around 1.3 - 1.4. However, deeper into the generation, especially after late 2015 when supplies started becoming more and more common, PS4 continued gaining lead over Xbox One consoles.
I think it's safe to assume, unless somethings dramatically change the future course, that we'll be seeing a very similar generation unfolding over the next few years.