About the manufacturers:
ASRock, MSI, Corsair, etc are all companies that produce PC components but not (really) sell any fully built PC. DIYers like us use those components to build computers so that you don't have to buy one from Dell or HP.
What makes people think that HTC and Samsung will provide lose parts for phones? This might just be a completely different market. Slightly smaller than the pre-built market as well, just like with PCs.
Panasonic, Nikon or Canon don't make smartphones, but I'm pretty sure they can figure out a way to make a proper camera module for a phone. Qualcomm and Intel don't make complete phones anyway, they just sell OEM CPUs. For all those brands it's not the loss in sales that everyone is talking about, but instead it's a gain in sales, product ranges and most likely profit.
All that said, I think that actually making this phone might be a lot more hassle and expensive than it needs to be. I seriously doubt that it's really going to happen the way it was shown in the video. Everything would have to be compatible, people would need to download their own drivers and install their own software.
As with PCs, the motherboard (main part) is going to get outdated over time and you still won't be able to put in that new faster CPU. What do you do? You get rid of the motherboard and buy a newer one.
The idea is just brilliant. We just need a way of properly executing it while remaining cheap and actually possible.