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!
There is no confusion. It has x16 lanes for graphics/slots, x4 dedicated lanes for NVMe storage, and x4 lanes for the chipset.
The chipset multiplexes up to x16 lanes of "stuff" onto the x4 chipset lanes from the CPU.
All of this is physically determined by the pinout of the socket and none of this can change unless AMD moves to a new socket. What did change is the speed of the lanes - x4 lanes on 4.0 is twice as fast as x4 lanes on 3.0.
AMD, like Intel, likes to pretend that chipset lanes "count" as full CPU lanes, arriving at a total of 36 effective lanes. But that's nothing new either.
That's correct, however, considering that Zen 2 supports PCI-E 4.0, that's double the bandwidth of the previous generation, it means that those multiplexed 4x can theoretically now support double the bandwidth of the previous generation 4x and it's not like the "stuff" that does get multiplexed over that 4x (USB, SATA, some 1x cards like sound, wifi and ethernet) also suddenly needs twice the bandwidth meaning that in practice, that 4x works as an 8x in Zen 2 motherboards. Great deal I say :)
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!