I currently am on my 5,1 2012 Mac Pro. I too wish they has launched an updated version of the 5,1 for around $3,000-$4,000 base.
I am still struggling with the decision on purchasing the 7,1. If it were about $1,000-$1,500 less for the 12 core, I would pull the trigger today (but probably more likely to go "up" in price?).
The cost of the Mac Pro is just the beginning for me - I have to buy pretty much all new everything to make the transition - TB3 External RAID, more RAM, Firewire card (since I have a wall full of Firewire drives with client files) Internal SSDs and even one of my 27" monitors needs to be upgraded!
Either way, I will have to maintain my 5,1 for all my 32-bit applications that are not being updated and are critical for my workflow - thousands of dollars invested. First time I have ever had an older Mac I could not sell to help offset the cost of a new machine - really a mess for an independent contractor. It is getting to where all I am doing is working to support Apple, Adobe, etc. moving forward!
The 7,1 is a work of art, but $700 casters is an I.Q test!