> The solution would simply be to stop making benefits decrease when salary goes up.

That's an option, and I'd be interested in how the math works especially with predictions of how the economy would respond.

That said I don't think not decreasing benefits is an important requirement. But net income (benefits + earned income - taxes) should be strictly increasing with earned income, and probably at a rate of at least 50% but ideally higher than that up to some reasonable definition of middle class income. It should never be the case that if you earn more you can end up worse off.

> That's an option, and I'd be interested in how the math works

The math works because the derivative of a constant is zero, it's that simple.

> That said I don't think not decreasing benefits is an important requirement. But net income (benefits + earned income - taxes) should be strictly increasing with earned income, and probably at a rate of at least 50% but ideally higher than that up to some reasonable definition of middle class income.

Exactly, it doesn't need to be fixed, we could also have decreasing benefits, simply with a much lower rate of decrease (ideally the rate of decrease should be the marginal tax rate for this income). But in practice such a system would be much more complex than a flat benefit system for little gain (the more limited the decrease rate is, the closer to a flat rate your public spending would be).