Craig Federighi talking to Matthew Panzarino:

“It’s only the M1 iPads that combined the high DRAM capacity with very high capacity, high performance NAND that allows our virtual memory swap to be super fast,” Federighi says. “Now that we’re letting you have up to four apps on a panel plus another four – up to eight apps to be instantaneously responsive and have plenty of memory, we just don’t have that ability on the other systems.”

And:

“We also view stage manager as a total experience that involves external display conductivity. And the IO on the M1 supports connectivity that our previous iPads don’t, it can drive 4k, 5k, 6k displays, it can drive them at scaled resolutions. We can’t do that on other iPads.”

And one more:

“We really designed Stage Manager to take full advantage [of the M1]. If you look at the way the apps tilt and shadow and how they animate in and out. To do that at super high frame rates, across very large displays and multiple displays, requires the peak of graphics performance that no one else can deliver.

I think Apple's arguments here are pretty reasonable. You could split hairs about the I/O limitations (could it be okay to only support up to 1080p or 4K on older iPads?) and you could argue that animations could be choppy and still be worth it. I would disagree, but you could argue those things.

What really puts this over the top for me is the virtual RAM that is enabled by the M1 processor, and appears to not be possible with the way that the A-series chips were made in the past. Let's say I open up a LumaFusion project that uses 3.5GB RAM. On an iPad with 4GB RAM, that's basically everything, so when I move LumaFusion to the background and open Safari and Messages, LumaFusion is killed so that I have RAM available what what's on screen now. That's reasonable because the app was physically not on screen anymore, so the fact that process was killed doesn't really impact my usage overall.

But when I open that same project in Stage Manger and then I drag in Safari and Messages, what should iPadOS do? It doesn't have RAM to do it all, so does it force close LumaFusion? Does it refuse to let me open new apps until I close LumaFusion? How does it communicate to me that I can't open an app because I don't have enough RAM available? Isn't this the OS for everyone?

Virtual RAM is the solution on the Mac and it's the solution here, and it makes all of this possible. It's not that you can have 8 apps open at once because each app takes up 1GB RAM and therefore an iPad with 4GB RAM would be able to have the same experience with 4 apps open, it's that none of this is possible at all without high performance virtual memory stepping in to make this possible in the first place.

Also, if I was guessing, I'd predict that the 8 app limit is not because iPadOS can only handle 8 apps at a time, it's because that's generally where they can know that even if the user has a few intense apps open, they can maintain the levels of performance that they expect without closing apps you can see on screen. You could probably technically run a dozen or more light apps at a time if this limit wasn't there, and I'll be curious to see what happens when someone inevitably tries to run the 8 hardest apps out there all at once in Stage Manager because I bet it can break if pushed too far. It will be fun to see!