Hydrogen vs. Battery Electric Cars

I don't know where I found this I forgot where but from what I read is that the grid can accommodate the increasing demand for electricity. Why do they always use the metric of when every car is all of a sudden changed into a BEV then they immediately say that the grid can't accommodate well in actuality the grid can handle two-thirds of all ICE cars turning into BEVs. Still, though, the cars won't magically turn into BEVs in an instant. Meaning the grid can upgrade over time.