What's missing here is "why" should the ones who are now able to ride on their parents insurance until they are 26 sign up for individual insurance? In a way, the very persons they need to sign up are the ones who they gave a waiver to.
Plenty of the "young invincibles" parents don't have insurance.
Hence the phrasing "... the ones who are now able to ...". Those who aren't able to ride on their parents plans probably can't afford individual insurance on their own without government subsidies as it is, so they won't be the ones providing the needed funding the legislation is banking on.
They don't need young people to provide the financing. They need them to provide a healthier risk pool, which will lower premiums, and thus subsidies, as a whole.