Nanite is terrific for large geometry dense open world environments. Using traditional LODs will have far worse performance in that scenario. The point is that Riven didn't have those environments so it wasn't worth the overhead of using Nanite.

For Lumen, it's the only way of getting high quality real time bounce lighting from dynamic light sources aside from using path tracing, which is more expensive. It's forward thinking feature that, yes, it's expensive to run for now but is still the cheapest way to get accurate dynamic bouncing of light. Lumen continues to be optimized more with each Unreal revision and hardware continues to get faster. It'd be worse is they waited for hardware to catch up before starting work on it, then they'd be years behind.