No, not a regression just a gimped progression. Thunderbolt 2 carries 20Gb/s, while version 3 does 40Gb/s and requires 4 PCEe lanes. GPU wants another 4 and so does the SSD for a total of 12. My guess is that in older MBPs Tb would take 2 lanes leaving 2 for the legacy breakout hub, but Tb3 needs all of them to support multi-monitor 4K and imagine the howling if Apple gimped that instead.

I tend to blame Intel for cheaping out - or scrooging - on the PCI lanes... hopefully Ryzen and the Meltdown blunder will make Intel more generous.