https://www.msi.com/Motherboard/support/B350-TOMAHAWK.html#down-manual
MSI B350 Tomahawk manual, page 18. You can have your GTX 1070 and your M2 NVME SSD at full bandwith no problem!
So sales of individual PC parts are doing fine, but overall "PC" sales declining. Does that mean that more people opt to build their own PC from scratch?
Honestly dude PCI-E 2.0 X 16's bandwidth is so big that modern graphics cards barely fill it, so as long as your cpu, ran and ssd is not bottlenecking you'll be fine!