I'm probably in a good place to answer this, as I've been desperately awaiting a proper replacement for my 2009 Mac Pro and yet I'm not a professional.
I use my Mac for writing music and use Logic as my DAW so am pretty much tied to the Mac Platform.
There's dozens of soft synth packages plus big sample libraries in my arsenal, so having expandable internal storage on which to store them is perfect for me and keeps my studio a lot tidier than it would be with external drives.
Soft synths get more demanding by the day, so having a Mac that's expandable in RAM storage (and even GPU) has been perfect for me.
I've had 3 or 4 drive failures over the last decade but haven't lost any data due to having the backups and replacing the dead hard drive has been a matter of visiting Amazon to order another and then copying the data back.
The BIGGEST mistake Apple could make is to believe that only Professionals need this option and that only professionals buy towers.
I've had towers since my 9600 Power PC, to G3, G4, G5 and then Mac Pros.
It was only when they introduced the Mac Pro that the prices started to rise so steeply.
The 2013 tMac Pro entry level price was $2999, that's $1000 more than the G5 was in 2006 - a $1000 premium in 7 years...ouch!
If Apple introduce a new Mac Pro starting over $3000 then it'll be out of reach to me regardless of how great it is.
Making an aspirational Pro Mac is good, but making an unobtainable one isn't. Introducing a Mac that ONLY Pros (or very wealthy) can afford would be a HUGE own goal IMO and would doom it to be a failure just as the 2013 model was.
There has to be a Mac Pro model starting at under $2500 for it to be a viable option.
If Canon only sold DSLRs to professionals they wouldn't sell many DSLRs. Having an affordable entry point for their range doesn't make them any less 'Pro' - a lesson that Apple would do well to learn.