The 50% increase is not required immediately. Only new cars would be required to be electric. Modern car longevity is about 10-20 years (Wikipedia suggests 200,

The 50% increase is not required immediately. Only new cars would be required to be electric. Modern car longevity is about 10-20 years (Wikipedia suggests 200,000 miles in the USA) so full capacity might not be needed for 20-30 years.