That's as ridiculous as suggesting low income earners spit out more kids just so they can get the benefits.
Noone earning $150k a year is going to keep on having kids just to get six months maternity leave at their salary. It might make it easier for them to do so, but it's not going to be a motivator any more than the current scheme is a motivator for someone on minimum wage (cos you know, that is replacing their income like for like - are they all out there breeding like rabbits just to get their 'free' money?).
This is about keeping women of all salaries in the workforce. Not the relative worth of SAHMs and working mums and low income earners and high income earners.