> Segmenting the grid is going to introduce a lot of trade-offs. Smaller segments means that dips and spikes in load are going to create greater differences in frequency, and as such require faster demand response to create the same level of frequency regulation as a larger grid.

Why wouldn't you make inter-grid (grid-to-grid) interconnects high voltage DC and use a DC-to-AC conversion to produce AC for intra-grid (grid-to-user) connections?