There is a little confusion whether it is 40 lanes or feels like 40 lanes. PCIe is a serial interface so they only need an additional wire per lane, per direction. So it is feasible as CPUs generally have spare pins.

As DDR5 is coming out next year, that will mean a new socket, limiting the upgrade path for the CPU, RAM & Motherboard. Although, 16 cores ~4.5ghz shouldn't be a problem for the near future (maybe 5 years even). Same goes with the PCIe bandwidth.

Edit: Just done some checking, I appears the 3950X has 24 PCIe lanes (16+4+4), but they are twice as fast, so not far behind the current 2nd generation ThreadRipper!