I was surprised by the 13.5M forecast too. I really thought it would be something around 10~11M for 2024's FY. I'm even more surprised by it considering that the units shipped this last quarter was below 2M

I don't think they'll hit 13.5M. I think they'll adjust it down to 11M by the holidays. That said, 11M would still be damn good.

Of course it's possible for Nintendo to miss the mark but certainly not by more than 1m. Why? 1. Nintendo has no reason to make ambitious forecasts for a console in its 8. year while the successor is already knocking on the door and 2. forecasts are harder to make during the strong years of a console's lifecycle as it's hard to exactly know how high the hype will really be, how long it will carry on and if really enough units can be produced to meet the possible very high demand but in the last 2-3 years of a console's lifecycle (before the successor arrives), naturally, it's way easier to make more exact predictions as there are clearly known facts (i.e. percentages of yearly declines) and less unknown facts (i.e. hype is gone, no production issues).

11 million would be only a few million less than the Wii U's total and only a little under half of the GameCube's total.

Heck, even a fluke at this point of 8-9 million would still be so darn good for such an old platform. 13.5 million seems optimistic and I think it will be more like 10-12 million. 

How wild is it that the successor to the Wii U is currently predicted on one of its outgoing fiscal years to sell nearly identical to the Wii U's entire life?

The Road to 160 million is likely, though not a given. We'll see how pricing and support after Switch 2 launches affects hardware sales. 

I see this 13.5 million forecast differently.  The reason Nintendo is making such an ambitious forecast is because they have to, and therefore they intend to do a lot to make this forecast happen. 

Why do they need to have an ambitious forecast?  Because Switch 2 isn't ready yet.  They want a strong software line up for the first 12 months for the next system, and they don't have that ready yet.  I expect the announcement to come early in 2025, and the release will be sometime in the second half of the year.  Since they only have 1 system, then they need to keep Switch 1 as profitable as they can for the next fiscal year.  They have to keep their investors happy.

Therefore expect the following:

1) Several software titles for the second half of 2024.  One of these titles has to be a decently big holiday type title.  It has to be at least as good as DKC or Tomodachi Life if not better.  There will also be several smaller titles and ports.  Pokemon Legends Z-A is a Switch 1 title that will keep the Switch selling in 2025 until their next system releases.

AND

2)  There is either a new hardware SKU for Switch or they are prepared to offer some serious bundles or price cuts.

So for the rest of 2024 and most of 2025 expect Switch to either get titles that would be made near the end of the system's life or games that can be put together in 1-2 years of development, ideally by devs outside of Nintendo proper: 2D platformers, Fire Emblem, a Torna-like expansion to XC3, Hyrule Warriors, ports, etc....  We should expect several more games like this before Switch 2.  Pokemon Legends Z-A is for Switch 1, because that comes from Game Freak.  The big games developed by Nintendo proper won't be coming until Switch 2 though: Mario Kart, 3D Mario, 3D Zelda, Splatoon, etc..., and that won't be coming until the second half of 2025.

I'm thinking Mario Kart 9 will be both a Switch and Switch 2 launch title hence the lofty projections for the physical year. Thinking both release next March.

Nintendo has adjusted their forecast by over 1M multiple times in just the last three years. FY 2022 (dropped by 2.5M) and FY 2023 (dropped by 3M). So it's not like they haven't adjusted forecasts recently. 

But I do think you have a point about how revising forecasts isn't really as necessary, even in the face of slower than expected sales, because it's at the end of its life. The units will sell eventually, the system isn't going to get substantially cheaper to produce at this point, and they will want to focus on manufacturing Switch 2 anyway. 

Maybe forecasts are high because of an incoming price cut? I still think Nintendo targets $350 to $400 with the switch 2.

I'm still not sure if it will outsell PS2 without price-cut, if Switch 2 is backwards compatible and $400 in March of next year who's going to keep buying Switch 1 in 2025?

The PS2 was price cut to $149 in the US in May 2004 (20 years ago). That is 3.5 years in with a price half the launch cost. Switch has never had a price cut and we are past the 7th year.

Nobody prior to the Switch unveiling could have predicted that a console not from Sony could surpass the PS2 while never getting a price cut.