Apple is a hardware company. What you're suggesting doesn't make business sense for Apple. They rely on being able to sell new hardware. Allowing upgrades wouldn't make sense. A laptop last for five years, for most people, being able to slap in extra memory, a new graphics card or just a new SSD would "rob" Apple of a sale of a new computer for an additional two or three years.
Somebody who will sell a high-end desktop system with: dual-socket Xeons, gobs of ECC memory, lots of storage, multiple 16x PCIe slots, and a very limited selection of high-end graphics cards.
That would satisfy the power users, and wouldn't cost them anything.
Sure they are losing customer, but compared to the hoards of students buying Apple laptops or iPhone customers, it not really an issue.
Latest problem I had were with the printer driver (Brother HL-1110 which doesn't seem to know that this printer has much less memory than advertised)
Where the Linux desktop is truly unparalleled is for a developer. As a developer I'm comfortable enough with the technical details to handle the quirks (like doing things from the command line), and the strengths of Linux truly shine (again, the commandline, as well as choices in DE, software, etc). One great example is Arch. On Arch I can install any developer tool directly from the command line with very little effort. Anywhere else and you have to click through endless menus.
If they licensed Magsafe to a clone manufacturer they would definitely lose sales. So many Mac users love the Magsafe.
It's really too bad they discontinued it.
I think they should give people what they want - a high powered upgradable machine. If they don't want to make it upgradable, then they should at least refresh it annually.
Having content producers using macOS is a huge benefit to their brand and I think it's a big part of driving sales of their iOS devices. At the very least, if they lose the content producer market there will be significantly more friction to producing iOS apps.
Yes, there's a halo effect, but I don't think it's all that huge especially with respect to video pros and I bet it mostly works in the other direction - people like their iPhone so they are more likely to buy a macOS laptop or desktop. Nobody is buying a Mac Pro because they like their iPhone.
The driver for iOS development is profit and as long as iOS users are willing to pay more for apps, it will continue to dominate. And if you want to make an iOS app, you are going to buy a Mac, but probably not a Mac Pro.
The Mac Pro is like the iPod in that it exists in this weird space where it continues to be sold but is clearly not something that Apple thinks is important.