No you don’t. It’s impossible for a Mac Pro to have for example a Lian Ali case. It’s impossible for a pc to have a Mac Pro case. The pure fact is that for waaaaay less than 6k you can get a PC that destroys the entry level Mac Pro.
I have a $4k PC that's my primary workstation, and it benches about the same as my base model iMac Pro.
But.... let's say things go crazy and I need to run quad GPUs in this PC a month from now, and also need 128GB of total video memory for 3D work. Can't do it. So if I want to do that, the $4k PC is now an extra $4K down the tubes, because I'd need a totally new system to do it.
I agree it's pretty silly to buy the base model Mac Pro with no intention to upgrade any of the components later, as you definitely have other options that make more sense (like the iMac Pro, if you want to stay Apple). But it offers capabilities on the high (very) end that you can't build yourself for cheap.