Suppose we increase the blocksize "so that we can onboard 1 billion more people." Ok, but then instead of onboarding more people we may just have the existing users increase their transactions by 10x. Then what? Fees would have to compensate and you again crowd out the newcomers.

Changing the blocksize ultimately doesn't improve anything and in fact can weaken the network. Unless we can prove that something necessary can't be done without changes we shouldn't prematurely change things.