Western countries and East Asia provide the highest quality of life for raising children, and yet have extremely low birth rates. What you're proposing is just welfare, and we already have that. Either way, contraceptives will likely not be limited or made harder to reach in western liberal democracies, except in the event of some kind of unforeseen crisis situation.
You're being vague and side stepping my point. Yes, the QOL is high, per most metrics people use to define it, sure, but hope is not high, and incentives still matter. Consider Germany vs France, two high QOL western nations:
France is much more generous about helping french people out with having kids. Germany is not. France has a higher birth rate. Yes, both have a bit of a problem with immigrants, but my point stands.
You can have a high QOL and blah blah blah, and find child care costs too goddamn much. Actual purchasing power due to the insane cost of living (RENT, Inflation, etc?) is rather lower. I know highly paid professionals who drop $3K/mo on childcare. I know people who aren't making that much up to their eyeballs in it.
Families have broken down, so a lack of grandparents to help is a problem.
Women work now, and more or less have to, since wages have been stagnant since the 1980s, so stay at home moms aren't economically feasible right now.
If behaviors are disincentivized, they will be expressed less than incentivized behavior.