I mean more would be certainly great, but with 20 you can at least run a x16 GPU and a x4 NVMe SSD, which is for my personal setup perfectly OK, I cannot do that with 16 lanes.
I'm hoping Ryzen 3rd gen bumps the lane count a bit more, otherwise Threadripper's 64 lanes look mighty nice...
I'm surprised they don't come with more, especially many of the current ones come with 2-3 x16 slots, AND 2-3 M2s.. Both Intel and AMD have been adding more cores recently. Plus, if you're doing any GPU-based rendering, they're going to simultaneously move data back and forth from NVMe to the GPU's memory. But it grabs a headline and most synthetic benchmarks will only test RAM, graphics and storage in isolation - sneaky!
Games definitely don't do this at all, which is the primary market for a discreet GPU in these consumer platforms.
When they do stream in assets they do so slowly & in a controlled, rationed amount to minimize impact on FPS. They are far from being PCI-E bandwidth limited. That's kind of why you see almost no FPS drop at all when restricting GPUs to x8 bandwidth, even.
If you're doing something more workstation-y or custom, that'd be when AMD & Intel would point you at the HEDT platforms which have more than 16-20 lanes.
In raytracing complex scenes can exceed your VRAM so if you don't want to fall back to CPU tracing you need a renderer that can swap parts of the BVH in and out on demand.
I'd imaging a workload like that or other HPC-compute workloads where the data set just doesn't fit in VRAM would certainly prefer more PCI-E lanes.
I think that'd usually be considered using the wrong hardware for the job but if you're just messing around as part of a hobby you're obviously not buying the $7000 Radeon Pro SSG, either.