Pro just stands for professional, and there are more than enough professionals for whom most modern systems are fast enough and offer enough extensibility. In that context, I think the term “prosumer” is pejorative because it is a portmanteau of professional and consumer. Why is a professional photographer or journalist or podcaster or digital artist any less pro than someone who needs a Mac Pro for work? Why is the type of computer you need for your work an indicator for how professional your output is? At least that’s what you are implying with this differentiation.
I totally get your point that there are still people who need a Mac Pro. For the past couple of years, they haven’t been served adequately by Apple. Apple promises that this will change in 2018. But late 2017 will already see the release of the iMac Pro, which is faster than most other desktop systems. It might not offer much extensibility, but it will undoubtedly eat into the Mac Pro’s market share. My point being: not nearly as many people are affected by the Mac Pro as one might think judging from Internet uproar. Apple’s other portfolio covers most high-end users.