If they were commercial failures then why are the Pro desktops with PCIe expansion slots updated in the past decade were released in 2012 & 2019 while Pro desktops without it were updated in 2013, 2017 & 2022?I think you forgot that the only Mac Pro without internal expansion is widely considered a commercial failure and Apple themselves have admitted that it was not successful. Mac Studio/iMac Pro class devices have their place and it is indeed possible that most enthusiast Mac users are satisfied with them, but it’s not what a potential Mac Pro user is looking for.
It is like saying the iPhone mini was a commercial success when it did not receive an iPhone 14 refresh.