The gap chart still says PS5 lead in the chart title.

No problem.
On the subject of PS4 taking the lead, I think PS5 production is probably high enough now that it should be able to retake the lead during the year. Whether it will be able to keep the lead come November/December will depend on how long the chip shortage affects production & whether Sony will be able to stockpile for holiday sales as PS4s 2015 holiday might be it's best.

Shortages bite hard for PS5, while XS holds up better versus its predecessors, presumably in large part thanks to the S being the only readily available 9th gen console over December.

I'm sure PS5 will catch up to its ancestor once stock allows

Xbox Series doing the heavy lifting in the Gen 8 vs Gen 9 comparison, tracking well ahead of its predecessor while PS5 is around on par with PS4.

Xbox + PlayStation
6th Gen - 182.33m
7th Gen - 173.21m
8th Gen - 167.37m (could reach 170m)

I wonder if the 9th Gen will be the first to see growth for those combined numbers?

I could see 9th gen being a growth gen if multi console ownership starts becoming more common. I could easily see a lot of customers saying to themselves “I really like my PlayStation 5 but Microsoft’s first party is too strong to ignore now” and just picking up a Series S on sale sometime down the line.

I don’t think the actual console player base will grow but I think everyone wins if more people just start owning multiple boxes instead of locking themselves into one

Just from a subjective standpoint, if you are at all interested in western AAA games output I don’t know why you would be on anything other than an Xbox or gaming PC as your primary machine from 2023 onward. You are going to have the entire Zenimax and ABK back catalog on gamepass, plus most of the EA back catalog and an increasing amount of Ubisoft. 

Unless you are a super fan of Sony first party or only play Japanese games the value proposition looks competitively really bad for Sony 

You've got to realize why these numbers are declining.  It is because Nintendo has taken away little bites during Gen 7 and Gen 8.  During Gen 7 the Wii ate into XB + PS sales during the early years, but then a lot of these people came back over to the XBox360 because of Kinect.  The result is that the Wii only took a small bite in the end.  For the Gen 8 consoles, the Switch took a bite into their sales, mostly during 2019 and 2020.  Neither of these systems had great tail end sales.  They probably could have avoided this problem by dropping their price below $300, but when the Switch is the same price, it looks like a better buy.

Expect Nintendo to take a really big bite this time around.  You shouldn't be looking for Gen 9 PX+XB to be up.  You should be looking for a big drop.

I'm sorry, but no.  Nintendo has nothing to do with this.  If so, the PS5 + XBS would already be significantly down from the previous gen LTD, since it's during the peak of the Switch.  But it's slightly up, with PS5s and Series Xs still selling out the moment they become available.  The Switch is a great console with great games, but its library is very different from PS/XBox so I don't think one replaces the other.

The main reason 8th gen is below 7th gen is that the pandemic forced MS and Sony to focus on 9th gen and stop making 8th gen consoles quicker than usual. This is the first gen since 6th gen, if not ever, that both consoles launched without problems or other issues that makes them less appealing. So I would expect both to sell at least as well as their 8th gen counterparts, and thus the overall 9th gen number to increase.

Yeah I don’t think Nintendo is really comparable to PS and Xbox anymore. You are missing so much of the third party library on Nintendo that they are functionally not even competing for the same audience anymore