Well no, it's also on society. Most governments want to incentivise productivity, particularly if the individuals have useful skills which they probably do if they can get into the 50k bracket.
And the point is that actually, lots of us don't 'need' to do it. If you think that, you've completely misunderstood. DH and I certainly don't need to. We're happy enough with that, the state might well prefer that we worked more and used our specialist skills, especially in this labour market.
Now this isn't necessarily an argument for increasing the 40% threshold, or any other actually. You can take the view that some people choosing to work less at the point when they decide it's not worth it to them is acceptable collateral. That's fine, but do so whilst fully understanding the situation.