I hear you, I had been wondering the very same thing, and actually have been looking at the very board you have, but I think I'm going to go with a cheaper Z170 so I can overclock.
I decided against overclocking right from the beginning. I am on a budget, I don't play games and this investment is long term.
Then if the TB3 thing gets worked out I'll switch out the mobo.
Like you I might need to change the mobo and hence probably the CPU too.
Yeah, it's the audio hardware I'm interested in as that's what I do. My interfaces are in need of an update, but I'd been thinking of going USB for that stuff anyway, for ease and compatibility.
AFAIK with music what's paramount is stability of transmission rather than speed. Many audio devices are FW 400 only. The issue with USB, IMHO, is its extreme dependence on CPU activity and its low stability.
The reason why it's so is that, unlike other technologies such as FW, TB or the old SCSI (1), USB doesn't have its own controller. That's why it's much cheaper. That's why it's unidirectional too. So what's doing the control? The CPU, what else? Once I had just bought a new USB HD and started backing up large amounts of data. My Mac became practically unusable: performances would regularly collapse.
So USB is perfect for cheap and not demanding devices such as keyboard, mouse, USB sticks, light back-up disks, cameras, etc., but when it comes to serious stuff like music and large amount of data it's not up to the task. With a technology like FW, the controllers on both devices (computer and external disk for example) communicate directly with each other and access whatever component it needs (hard disk on computer for example) without interfering with the CPU.
If I had to replace my audio interface I wouldn't choose a USB one even though you find practically nothing else today. I would go for a used FW one.
Nick
1) I still own SCSI devices! they are just gathering dust on a shelf of course.