Xbox Series is leading Xbox One by 1.5m and 360 by nearly 4m in spite of being severely supply constrained (particular Series X), and yet we still have people predicting lifetime sales for Xbox Series in the neighborhood of 50-60m, lol. Xbox One's best chance of catching back up to Xbox Series was on the Holiday since XB1 was always a strong Holiday seller, and while XB1 did make up more than 500k of the gap in November, Xbox Series was gaining again in December thanks to stronger Series S stock in December, and now for the first 2 weeks of January Xbox Series has been leading XB1 by about 60k, which establishes that the Xbox Series 2022 weekly baseline will be well above Xbox One's weekly 2015 baseline, and this is while Series X is nearly as hard to find as PS5 is. Eventually supply will catch up to demand, and eventually the dozens of 1st party games Xbox has in development from their soon to be 30+ studios and 40+ teams will start releasing.

I feel like the Bethesda and Acti-Blizz acquisitions pretty much guarantee 70m+ for Xbox Series, and I'd say that topping 360's 86m is a likely possibility. 

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.

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.

Switch was only ever behind due to the different holiday timings of the two consoles. There's no perfect comparison for two consoles that launched during different times of the year, but I maintain that 12-month-intervalls remains the best way there is, because that gives each console all available months of the year exactly once.

This then means that Switch always led with a clear trend of growing its lead.

Year 1: ~2m ahead
Year 2: ~4m ahead
Year 3: ~6m ahead
Year 4: ~14m ahead
Year 5: ~20m ahead

Switch vs. PS5 comparisons will look very similar. The differences will be that the PS5 is a bit more frontloaded than the PS4 and then won't peak as high as the PS4 because Microsoft is putting up more of a fight this time around.

I think there is growth possible, and I can explain why. A reason why often the numbers of XBox+Playstation are added for these gens - and not Nintendo - is because Sony and MS are basically carrying out the same strategy and targeting the same market for over a decade. Nintendo instead was trying out different stuff, targeting different customers. There success varied based on the customers they targeted.

This gen a few things change. First of all, even a classic strategy may see growth, as more regions get wealthier and tech gets cheaper, so more regions may be stronger into buying luxury products like games. The overall customer base is growing. I see that happening mostly in Asia, but also south america. The second reason is, that MS changed their strategy. They target Gamepass now, and that may allow them to find new customers, even in classic market regions. I am not sure though how much of them convert to Xbox, or if most of them stick to PC or streaming.

What you are saying sounds reasonable.  And, if Nintendo has no effect on PS+XBox, then I would agree that Gamepass is the kind of strategy that could grow the market (and so would expanding into new territories).

However, I do not accept this idea that Nintendo is targeting a completely different set of customers.  The Switch is going to be Nintendo's most successful system of all time.  It will be their #1 system when it comes to profits and total software sold, and on hardware it will be either #1 or #2.  It is clearly reaching a lot of customers that it did not reach a generation ago with 3DS + Wii U.  Are 100% of these customers brand new to console gaming? 

A person can look at Japan and see that this is definitely not true.  Nintendo has taken over the whole country's video game market.  Switch is clearly competing with Playstation in Japan.  Likewise, other Eastern markets have similar tastes to Japan (China, S. Korean, Taiwan, etc...).  It is foolish to think that Switch isn't competing with Playstation in these regions.  A person can look at the top 5 charts from S. Korea and Taiwan and see Switch games dominate for most weeks.  The idea that Switch is not competing with PS + XBox is clearly wrong.  Furthermore, if Switch is competing with Playstation in Japan, then the most reasonable conclusion is that it competes with Playstation in every region.

In the Americas and Europe, Switch is also very popular.  It will not take over the entire market like it did in Japan, but it will take a % of the market in these regions.  Switch's popularity cannot be denied.  And this belief does not make sense, "Switch competes 100% with Playstation in Japan, but 0% in every other region."  Switch competes with XBox and Playstation in other regioins too, it's just not an overwhelming victory like it is in Japan.  However, Switch is definitely gaining ground in other regions, and that is why it is reasonable to think it will eat into XBox + PS totals in these regions too. 

The Switch's new customers come from Playstation.  That is where they came from in Japan.  That is where they are coming from in other regions too.  It's not quite obvious yet, because these early PS5 sales appeal to the top of the market, the people willing to spend the most money on gaming.  The top of the market always buys new consoles first.  Once Playstation gets to the point where it would sell to the middle and low ends of the market, it will be obvious that Gen 9 sales are going to be down for PS + XBox.  In fact, even in Japan PS5 hardware seems to be selling ok, even though software sales are abysmally low.  We can expect that PS5 hardware sales cannot continue to do well in Japan.  We should also expect PS + XBox sales to fall short in the rest of the world.  It will probably take a couple more years for this to be clearly seen in the data though.