I don't think the 2013 MacPro was a mistake as a Mac. It was a mistake as a replacement to the older MacPro, but it could have been a great Mac if it was designed as a mid-range Mac, below the tower-based, modular MacPro. I love the quietness of my Macs, and having a Mac more powerful than the Mini but less than the MacPro would have been a great Mac, because it's a design with a single fan. But replacing the modular MacPro with it was a mistake.
Of course a mid-range Mac is out of Apple interests, because it would compete directly with the iMac, and Apple wishes to sell a lot of iMacs (AIO machine, all components even the display in the same enclosure, so if any component fails you can easily convince the customer to buy a new Mac... that means more income for Apple).