Hm, USB-A and SD aren't proprietary, so I guess you're considering them legacy? SD certainly hasn't been replaced, and I've yet to see a memory stick that isn't USB-A. Ditto for hard drives, though I'm sure they exist.

I understand that thickness perhaps knocked out the USB-A, but the same can't be said for SD. And since the ultra-thin keyboard hasn't worked out that well, then perhaps the MBP is a little too thin for its own good?