You did not understand what I meant; with a MPX card, any USB-C port (excluding any additional ports added by other PCI cards) on the machine can potentially be used as a display output. Non-MPX cards can only use the display outputs built into the card. Now I'm not saying this is how I would normally operate but it is indeed a feature present on MPX cards. Although currently I am using a thunderbolt output on the I/O card. Without a MPX style card you can't drive any displays from the I/O Cards 2 USB-C/Thunderbolt ports. And maybe you do not need to, great! For certain things I need to do, I need that ability. Currently I have 6 displays connected (4 4K@60Hz, 1 Ultrawide 3840x1600 and 1 1080p.) I am going to be changing this up a bit though since I have shifted most work to my M3 laptop. More than likely going to put my 580X GPU back in so I will have that and the W6800X Duo; which will work better for the instance I need all those monitors active.
Yes getting powered by the MPX slot is a feature because it simplifies installation and did not require me to add cabling inside the tower.
I understand the reasoning behind not getting a MPX module because of the cost involved.
But this thread is an argument about spending money *right now* on something that is basically a dead end. Why would you spend money trying to upgrade the CPU (which is going to be expensive unless you buy used, and a pain in the ass to install); instead of investing in Apple Silicon which has a longer future and is faster.
Perhaps the OP has a specific reason on needing the 2019 Mac Pro which has been left out from the original post.