It seems to me that a bigger issue is lack of storage. With battery storage acting as buffers we would normalize/smooth out required transmission rates.
But storage has dropped in cost over the last 10 years, by a factor of around 5x.
And with cell chemistry that is more resilient to charge and discharge cycles (LFP) coming to fruition in the last 5 years (and at a lower cost than NMC) it makes sense that we're only now seeing large-scale investment - the yield curve has hit the sweet spot.
There's a nice diagram of the Scottish side in this article:
https://www.zenobe.com/news-and-events/blackhillock-battery-...