Wouldn't the surplus of selling relatively few machines that cost a minimum of 5,999 US$ justify continuing its development?

Not a burn, they're clearly great, well built computers, but Apple obviously adds a lot of profit to all their machines, Mac Pro included.

I think the Mac Pro exists for reasons independent of directly making a profit. Though Apple would always like to make a profit, of course.

It's a combination of a halo car and a triple whopper.

A halo car is a statement of intent from a car company. It's showing off "here's all our coolest stuff, and the direction we think things should go", while being impractically expensive and bought by almost nobody. By putting out the fancy new Mac Pro, Apple reassured professionals that it cared about them, and presumably that's making people less likely to nervously shift away from the platform even if all they actually need for their own work is a laptop or Mac Mini.

The triple whopper, famously, mostly existed to be a conspicuously more-excessive product that made the double whopper look more reasonable in comparison. If the Mac Pro exists at $6k base, buying a reasonably specced out iMac suddenly looks downright responsible!

Both of these things are good for Apple's bottom line.

The other side is that if the efficiency and performance of these chips scales up to a big workstation, they could use them in their data centers to get much higher performance without a whole lot of extra power, and then sell whatever chips they don't need as Mac Pros.

I have long believed the Mac Pro is a marketing tool that they could happily sell at a “loss” — Using quotes because the difference could be considered a marketing expense.

Given the price it likely is sold for more than cost; I’m sure the pricing is marketing driven too. It doesn’t just drag the top end of pricing upwards (the classic cheap/middle/extravagant split that drives buyers to the middle) but sends a message. The $1K monitor stand is a good example,

In this theory, it’s important to Apple that something be almost always available in the “Mac Pro” zone, but it’s not important enough that they need to be diligent about it.

They have this same phenomenon with the super-expensive watches (though I believe the $10K models are gone): basically identical to the cheapest one, and “wear out” (stop getting sw updates, no battery replacements, etc) in the same time too.