Personally i don't think Mac Pro is SO outdated. Of course the internal specs are super-obsolete in the Workstation PC market. And today there's no reason to buy a New MP for no reason, if they don't at least cut the price half. But the concept of nMP is completely different to what every professional means.
The 13 Pro model is a modular machine. That is what Apple wanted it to be. Cupertino didn't mean to use and abuse the internal capacity of the machine, they wanted to exploit the possibilities of the computer: you can upload just the CPU, and please do no tell me that a 12 core v2 Xeon is not enough, because it is. 12 cores are unnecessary to the "Pro" market Apple targets with Mac Pro: artists and engineers in the multimedia world. The amount of plugin is it possible to open, used, being stable with Pro Tools even without DSPs is ridicolous. I'd say overkill. And the number of effects you can load is phenomenal even without accellerators. Also, 6 TB2 ports means 36 different devices. Do not complain on the D300. You can buy an eGPU and refresh it as many time as you want. You can slam as much GPUs you do need. Internal storage? Do you really save files on a SSD? Come on, we all have at least a Portable Hard Drive. And with 200 bucks you can build a NAS. The whole apparatus of Desktop PC is in decline. Do not tell nobody, but buying a 15' rMBP 2015 version is still way better than buying a new 13' nrMBP. That is not because the old is better, is because we are not so used to Thunderbolt nowadays, still too much expensive, still unexploited.
And the nMP has TB. In my honest opinion, if i have to buy a finished workstation today, i'd still buy a nMP over ALL the competition. As i write the post, i am looking at the HPZ840 and, yes, for 4400 dollars they offer a 1TB 7200RPM HDD.
Buy the 4-cores model, a new CPU and eGPU and, that's it.
Not saying that an update is unrequired, matter of fact it is, but do not compare assembling your own pc compared to buying a finished one. It is like comparing a DIY scarf to a retail scarf. Little unfair comparison, isn't it?