How much did you pay for your 5K display and your OS license? If you’re going to compare your build against an iMac or iMac Pro you need to be honest and have a fair comparison.
As others have pointed out elsewhere in this thread, the iMac Pro and past Mac Pros have been quite price-competitive with equivalent offerings from other vendors like HP and Dell. Mac Pros do not cost “three times” what an HP workstation costs. It’s not a tricycle/Lamborghini difference. Not even remotely close to that.
If we talking about consumer grade computer, I think self built computer are more competitive than regular iMac, depending on individual purpose (I am talking from view of indie creator)
iMac cramped enclosure tends to overheat cpu under heavy load and will kick their fans like jet engine, only 27 inch offer user accessible memory slot, the rest was completely sealed. Buying from retailers is also PITA since their tends to carry Fusion / 5400 rpm spinners as default configuration, unless BTO directly from Apple with SSD only.
In the past my buddy iMac GPUs was burned (27 inch, 2011 model) causing loss of working hours. Sending back to Apple was hassle and need about 1-2 weeks depending damage. To add insult to injury what happened if this happen in country without actual Apple Store? (only retailers)
With DIY computers, when one components fails, just run at nearest PC store and replace them. No longer time waste
Not everyone need iMac 5K display, e.g music maker doesn't take advantages iMac DCI-P3 color space, coders who already had solid multi monitor setup, or digital artist who prefer clinging with their 94-98% Adobe RGB Cintiq Pro.
For current iMac Pro, their price is competitive then yes, I agree. They cannot be replicated in any custom specification from DIY side, similar grade Xeon W chips based workstation currently only available from HP / Dell but it not run OS X.
While Mac Pro doesn't cost three times from HP counterparts, they tend have higher specs and offer things which not available from Apple, e.g. actual workstation class GPU with certified drivers (Quadro/RIP FirePro/Radeon Pro) taking into consideration. Even on most powerful Mac we only have clocked down version AMD cards, unless you throw full fat desktop GPUs in older cheese grater pro or eGPU enclosure in newer TB-3 based Macs.
Not everyone in same boat like OP, so I'am quite understand his position. For 'Who' this Mac Pro?
- Some pros might ditched old machines and simply buying new from Apple when they needs new Macs.
- Some pros are prefer have greater control of their machine while simply replace /upgrade hardware their needs.
Folks in number 1 category will bring iMac Pro to their studios, folks in 2 category mostly wants updated version of cheese grater. 5,1 was proof from decade ago: upgradeability was able to keep computer live longer and even compete with modern Macs in some aspect. But Tim doesn't hear
Just my 2c.