If it's backward compatible, than what would an adapter do? If it's a power adjustment than it's not 'entirely' backward compatible. If it's a pin thing, than of course they could just supply cables. If it's a data thing, than it's simply not truly backward compatible.
I think you're making much ado about nothing here.
USB-C is basically the port and cable specification. TB3 requires hardware and software behind those ports. To be TB3 'compliant,' think of it as it MUST be able to work with TB1/2 devices and not only TB3 devices.
Sure, there are differences between active or passive (e.g. pin/format swaps only) cables, but the end result is the same. Keep your existing devices, run a cable that plugs into a TB3/USB-C port and done.
Yes, there is AFAIK a cable difference - think of USB-C as the connector only, and depending on the electronics and cables,
may support either USB 3.1/Gen 1 (~5GB/sec), Gen 2 (10GB/sec), or TB3(bi-directional and supports DisplayPort and PCI-Express, 40GB/sec). Of course, you also have backward compatibility both back to USB1 + TB1 or 2 assuming you are using USB-C TB3 ports and the right cable.
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Each pair of thunderbolt 3 ports requires a separate Alpine Ridge controller. The first one comes from Skylake CPU, the second one must be an extra chip on a logic board.
Unless Apple really wants to reduce confusion, I highly doubt they won't try to save a few bucks by not using an extra chip (you've all seen how they skimped on RAM in iPad "Pro" 9.7")
And 4x thunderbolt ports seems like an overkill, really
Yeah, this is the interesting question - it's possible we see two ports on the left and two on the right and e.g. left ports are TB3, right are USB3g2 only, or all in across all four. I'm not seeing either path as a huge impact to me, as you can always use the right ports, for example, for connecting to projectors and external displays on the go, using USB-C -> DP or HDMI regardless, and I'd use the left for 'docking and power at desk' or similar.
