You're just making things up. Once that child grows up it's no longer a "pet project" and contributes to the continued existence of society. Spending resources on the child is an investment into the future productivity of that child when it enters the workforce. The idea that such a logical investment is considered uneconomical is a fatal flaw in modern society.
When you consider that the type of family (highly educated) that can actually afford to have children is deciding against having children then your proposal to consider every child as a hobby is just plain stupid. Who's going to have children then? Not the upper class? not the middle class? Not the poor? Nobody?
Well, if we are talking about poverty and inequality, the child may be a net negative for society, even on the ongoing basis before you consider the initial investment.
Plenty of people are still going to have children, anyway; if we at some point find ourselves short of children, we can bring the incentives back (or just have more immigration).
But I don't think it's likely to happen over the long term... the days of needing to make the maximum number of factory workers and cannot fodder for the motherland are over, at least for a time. Having children is totally a hobby.
When you consider that the type of family (highly educated) that can actually afford to have children is deciding against having children then your proposal to consider every child as a hobby is just plain stupid. Who's going to have children then? Not the upper class? not the middle class? Not the poor? Nobody?