Why is it irrational? in a way it is creating liquidity (the additional funds will be spent in different ways by those in different socio-economic groups). Why

Why is it irrational? in a way it is creating liquidity (the additional funds will be spent in different ways by those in different socio-economic groups). Why does there need to be a cut-off the whole point of it being Universal for it to work, there should not be anyone just above the cut off as its in addition to their existing salary, this is a problem that exists in the current welfare states, where those earning just above cutoff earn similar to those on welfare.