I would say that one reason why se saw the X360+PS3 to X1+PS4 decrease is because there was a lot more reason to own both consoles in the X360 era than on X1/PS4 and now, it was a much better competitor to PS than any other console have been in the 5 gens mainly because of the exclusives that would have even people that are PS fans buying the console.
It is also worth pointing out that the Xbox 360 had one more year on the market than the PS4 & XBO did before replacement. That does help Gen 7 since it was such a back-loaded generation for PS & Xbox.
Also, the legs of the PS4 and XBO were cut out from under them. While there was a spike in demand for them in Spring 2020, stock inventories were depleted well before that year was out. Sony and MS were likely unable to make more of them because the same factors that generated increased demand for consoles also impacted supply chains. They were right at a generational transition and thus likely had to fully invest in production of the PS5 & XBS. MS effectively stopped production of the XBO at the end of 2020, though Sony kept producing small amounts of the PS4. As a result, while the PS3 & 360 sold over 2M in 2014 and another 921k in 2015, the PS4 & XBO sold only around 600k in 2021, almost all of that being the PS4.
By the end of 2019, the PS4 & XBO were still leading the PS3 & 360 in launch-aligned terms in the U.S. If their sales from 2020 to now were the same as combined PS3+360 sales for the 2013-2015 period, they would have sold close to 70M in the U.S., same as the final total for PS3+360, instead of the 64.4M they actually sold.
Worldwide sales, though, were about flat gen-over-gen, so the deficit in the U.S. (and a smaller deficit in Japan) was made up for by gen-over-gen increases elsewhere. That further implies that had nothing disrupted supply & demand dynamics (including production), combined global PS4 & XBO sales would have exceeded those of the PS3 & 360.