Apple could treat the Mac Pro the way the Volkswagen Group treats development of the Bugatti lineup — a loss leader that attracts attention and spurs the development and testing of new, cutting edge technology.
Yes, but economies of scale make that impractical.
That used to be how it works — things start at the Power Macintosh and trickle down to the LC. But now, most new SoC features get introduced at the A-level (iPhone), M-level (iPad/most Macs), or M Pro-level (higher-end Macs). For example, the M1 Pro introduced better video codecs that eventually trickled down, and were of course also added to the M1 Max and M1 Ultra.
Building a Bugatti factories is several orders of magnitude cheaper than building the factory capable of producing tomorrow's chips. Apple is able to have TSMC (and others) manufacture such impressive chips because Apple has the volume. 200 million iPhones a year, plus tens of millions more iPads, Macs, Apple Watches, all on the same basic building blocks.
So you'd have to make it up in volume, and the Mac Pro does not have that volume. It never will again. It's like offering customers the Apple CRT Display. Almost of all of them will be baffled why they would even want that.
But, as @CWallace says, that could change with heterogenous process nodes. If portions of the M5 Extreme are manufactured with the same process as the M5 Max, but then a fancy new feature is manufactured with a worse, larger efficient process, and enough people buy the whizbang feature for Apple to still benefit (such as by getting real-world data on how it's used) and eventually trickle the feature down to other devices, that might be an option.
This sort of work would grant them supremacy in chips. There would be no advantage (except maybe cost) in choosing a machine with an AMD Threadripper over a Mac with the flagship chip — especially with the ARM versions of Windows and Linux that now exist.
There's plenty of market segments Apple has no interest for, such as putting a Mac in a data center. You'll be better served with other vendors there.
It’d be great if Apple could revive Bootcamp, too. I prefer macOS, but there are certain applications and certain tasks that still call for either Windows or Linux. I’d prefer to run it all on the same hardware.
Asahi Linux does exist, although I believe they don't have the M4 running yet.
As for Windows, I believe the challenge, drivers obviously aside, lies mostly in the boot process. Windows's hardware abstraction layer (HAL) assumes either BIOS or UEFI (presumably, the Windows NT versions for PowerPC also supported OpenFirmware?); most ARM devices offer neither. For Qualcomm Snapdragon, Qualcomm has added an UEFI-lite layer of sorts that's just barely enough to get Windows to boot, but, as Linux developers have found, isn't enough to treat them as UEFI-capable chips. For Apple Silicon, iBoot is similar to OpenFirmware, like on their PowerPC Macs of the 1990s and 2000s.
Someone would have to adapt this, and neither Apple nor Microsoft seems to care enough.
What's in it for Apple? A bunch of people who say they'd buy a Mac if they had this additional ability. Sure. But that's not a whole lot of people. Most of them can just buy a second device.
What's in it for Microsoft? More Windows users, which hasn't been a growing business for them in a decade.
So why would either of them do it?