What are you talking about? You initial statement was that these are XCode servers for CI. (I presumed that CI is continuous integration/improvement
https://developer.apple.com/library...onceptual/xcode_guide-continuous_integration/ ). There are two parts to testing continuously. One, is building something new to test. Second, there is doing the actual testing. Those don't have to be the same machines. In fact, there are lots of good reasons why they shouldn't be. Separating the compile/build from testing is same class as why the source code repository ( Git , Subversion) is a separate set of servers (and services ).
Testing what Apple is building every day should
not be done on homogenous machines (one single product) . Even more so should not be done on machines the over whelming vast majority of Apple users do
not have. Apple should be testing on the machines the users use, since that is where the software is primarily going to be used.
The "headless" machine can do the source merge , compile , link, build sequence to compose the various builds Apple needs to ship out to the machines to actually do the tests on.
screw some "Simulator" Apple should be testing on real stuff. Millions of folks are not using simulators. It would to go explain Apple's declining software quality levels if they are primarily targeting simulators for testing.
server side web services ... that is where Apple is going. There is all kinds of bright, flashing neon signs that indicate this.
Again this is primarily the context of "headless" server zone context. You want to move the goal posts out of that context fine, but that target Apple does not have a "no option than to keep making the Mac Pro" constraint at all.
Which should be tested on the machines that those millions of user actually use.
A Mac Pro is not absolutely necessary to write/develop code.
Once again the Apple Design award winners "behind the scene" photos have not a single Mac Pro visible.
https://developer.apple.com/design/awards/
It is nice, but it isn't absolutely necessary. Apple software ecosystem development would not immediately crumble if there was no Mac Pro. The Mac Pro has huge systemic problems if it is a one-trick pony. It isn't a viable product in that case.