And worst thing that could happen to People owning first gen Mac Pro.
The worst thing that could happen to first gen "new" Mac Pro owners is that it became the last gen Mac Pro.
That would be worse.
A $25-50 dongle to go from miniDisplayPort based connector to Type C based connectors isn't the end of the world. Expensive but not the end of the world. TB v2 only bandwidth cables should be substantively cheaper than the TB v3 ones.
You are also presuming that there is a rapid and massive shift to the new Type C based connector for TB devices. I wouldn't bet on that. Vast majority of Type C connector devices coming to the market are going to be USB only with no support for the alternate modes. It is cheaper so the "race to the bottom" USB market is going to largely follow that path.
The TB v2 design knowledge is more mature and more broadly spread out. If Intel sells the TB v2 controllers at a deeper discount I suspect there will be more than few peripheral vendors that follow TB v2 for another couple of years.
TB v3 is gong to have an even longer
and deeper Intel certification/validation testing sequence to go through. The port is going to have to support TB v1, 2, and 3 along with "legacy" (from TB perspective) DisplayPort and USB (for a subset of implementations) pass thru modes. The Type C connector was designed with 20Gb/s in mind so the stretch to 40 Gb/s is going require attention to detail ( not the cheapest chop-shop components you can find ). Before had PCIe , DisplayPort, and GPIO inputs. Now probably have a fourth of USB 3.1.
( the
https://thunderbolttechnology.net/blog/thunderbolt-3-usb-c-does-it-all mentions there is a 10GbE direct point-to-point mode now. For systems that could be another layer in the cert process. Wonder how much hardware assist is in there. )
Using Type C doesn't mean TB devices will become "just as cheap as" USB devices. Not going to happen.
Very narrow dongles like FW , Ethernet , don't buy much more with a TBv3 controller as TB v2 was overkill for speed already.
The higher end of the TB market will certainly move up quickly, but the parts of the market that were trying to work closer to the affordable range probably are not. Just like there are way more USB 2.0 devices than USB 3.0 ones... same thing will be relatively true for Thunderbolt for at least a couple of years.