The "trashcan" Mac Pro (late 2013) was my favourite Mac of all time - it actually fit my professional needs perfectly, and performed incredibly well in a small package for a reasonable price (I couldn't spec out a comparable Xeon system at the time that was cheaper). I don't think it could be considered a failure by any means. It was a sweet spot when it came to price/performance for many of us in tech/science industries, and perhaps that was the reason it wasn't replaced for a long time.