You’re conflating two things. The 99th percentile design temp for winter heating systems only implies a steady load would be imposed on the grid. Optimizing HVAC systems for the 99th percentile load is proper engineering. A higher powered electric resistance heating plant would make the grid problem worse right now (as houses could draw enough energy in a day to maintain temp rather than only drawing enough energy in a day to not quite maintain temp).

It doesn’t mean you design the grid to only sustain the loads 99 percent of the time. This is going to turn out to be a grid implementation problem, not an insulation or a renewables problem.