> The trouble with renewables is that a lot of the drops in supply are both unpredictable and relatively long, which makes them hardre to deal with.

Not so; when the problem is bad weather rather than the diurnal cycle, even weather forecasting gives more forewarning than emergency stop buttons in power stations, and (given the geographical distribution) the change itself is smoother. On the scale of a continental grid, do you even need to care about anything besides diurnal storage/demand-shifting (as preferred) and seasonal issues?