When infant mortality goes down and Children are expensive (need long educational investment) instead of being helping hands in low skilled menial tasks then population growth tends decrease. Modernity reduces the advantages of large families, in fact in some cases modernity causes huge disincentives to large families.

I think it's viable but I also think you could either make them one third the weight or three times more HP and torque , also you could use carbon fiber wheels and low weight tires - https://nironmagnetics.com/#technology , http://curtran.com/our-products/magnetic-wire/ , https://insideevs.com/news/318031/breakthrough-insulating-material-lets-electric-motors-perfom-hotter-than-ever-before/

Give me a car with a 100 miles battery range (around 30kwh) and a 25kws of fuel cells and a tank that lets me have 350 more miles or how about a car that goes around 150 miles on batteries and has a built in hitch and connections that lets you attach a trailer with 35kws of fuel cells and a big enough tank to go an extra 700 miles- you can see the possibilities, you could get away with hydrogen refueling say every 300 miles or so, people would do the majority of their driving on cheap electricity and a couple times a year take long drives powered mostly on hydrogen.

Stronger than pure aluminum but there are stronger aluminum alloys out there, my guess is that the cost must be the breakthrough.

A good reminder that while the energy density of the cell is very important, that in the end it's the energy density of the whole energy storage system that truly matters.