So is that the public just isn't interested in PS4/Xbone anymore since they're old and replaced, or are they just not manufacturing many of them any more, perhaps to free up as much production capacity as possible for PS5/XS?

I would say it's a mixture of both. The chip shortage affects everything so production lines are going to be heavily skewed towards the next gen systems. The X1+PS4 community is approaching end levels of 360+PS3 so market saturation is getting there for the older systems. Add in the fact that both last gen systems are still priced pretty high for this time period in their life cycle and never received that last final price drop they deserve and you get weekly numbers like this. However, PS4 did ship 1M units in the last quarter so we'll have to wait till the end of the year to get a clearer picture of how low Sony at least is actually selling for last gen numbers. 

So PS5 to hit 10M before XS/X hits 6M?

I would say those are two-thirds of a whole. So yes to both, and then we can add the final third of the equation being the price. The PS4 is still priced in such a way that it makes it stupid to get one instead of a PS5.

The damn PS4 pro STILL costs $399.

That will buy you a PS5 digital if you can find one.

I would guess it's a bit of both, but actually depends on the region. Here where I live, the current situation is that retailers don't feel the need to restock PS4/Xbox One supplies anymore because the demand is simply too low to make it worth it. But PS5 and Series X/S is too supply constrained that it's incredibly hard to get them. I also live in the country where console gaming is incredibly niche overall, so these consoles as far as I know don't get restocks too often. Especially Xbox, which was sold out day one and then got another stock only a couple months later. So, yeah, basically the Switch is the only console you can really buy here at this point.

Yeah I forgot a PS4 Pro is the same price as a PS5 Digital and PS4 is the same price as a Switch, both of which are much more in demand. Xbone is cheaper but it was never a hot product to begin with.

I kinda thought more people would be getting one as they're unable to find their next gen replacements, but I guess we are getting close to that roughly 175 million combined number that PS3/360 stopped at and after 7+ years you'd think anyone who wanted one would have one by now.

Or maybe both consoles have limited production and they are selling whatever it’s produced, combined with the high price they are selling them.

In 3 months, the PS4 slim and PS4 pro will be 5 years since they launched. Not a single price cut since.

I don't think there'd be much demand for them now that the new consoles are out and they've been out for over 7 years, at least not enough to be widely sold out.

PS4 Pro was only discontinued in Japan, not globally.

And what is their motivation to drop the price to $199 when their focus is now on PS5 and they're probably turning a very healthy profit per unit at $299?

That is not how it works. The fewer "things" you manufacture, the more expensive they get. PS4 production levels are low now, and with the semiconductor supply problems, it costs a lot more now to manufacture the thingies, despite R&D having been recouped fully. If anything Sony taught us with the PS4, they are not willing to lose money on its hardware.

My guess is someone inside Sony already came up with the idea of killing PS4 production (one final big shipment) in favour of the PS5.