Nanite is a lot more than just a continuous lod system. The challenges they needed to solve were above and beyond that. Continuous lod systems have been used for literal decades in things like terrain. The challenges for continuous lod for general static meshes are around silhouette preservation, UV preservation and so on. One of nanites insights was that a lot of the issues around trying to solve automatic mesh decimation without major mesh deformation/poor results just disappear when you are dealing with triangles that are just a few pixels (as little as single pixel triangles) in size. The problem with small triangles is a problem called quad overdraw, where graphics cards rasterize triangles in blocks of 2x2 pixels, so you end up over drawing pixels many times over which is very wasteful. So the solutions they came up with in particular were:

- switch to software rasterization for small triangles. This required a good heuristic to choose between whether to follow the hardware or software path for rasterization. It also needed newer shader stages that are earlier in the geometry pipeline. These are hardware features that came with shader models 5&6.

- using deferred materials which drastically improves their ability to do batched rendering.

It's actually the result of decades of hardware, software and research advancements.

The 2 solutions posted in recent days seem heavily focused on just the continuous lod without the rest of the nanite system as a whole.

Also yes, there were also challenges around the sheer amount of memory for such dense meshes and their patches. The latest nvme streaming tech makes that a little easier, along with quantizing the vertices which can dramatically lower memory usage at the expense of some vertex position precision.