As well as reducing population, negative birth rates have the side effect of ageing the population.
A population where most people are over 65 is one where ever more resources get spent on healthcare & pensions, paid for by the ever fewer number of working age people.
There’s also various toxic political effects ageing electorates can have on democracy.
There is a very rational argument for the population being expanded: we need the population pressure to push us to invest in launch cost reduction, space mining and colonization so it becomes feasible to move heavy industry and eventually the population into space to leave earth to recover. I would rather see earth become a pristine zoo than a tomb and reducing population only cuts a small subset of the risks of becoming a tomb and exacerbates others.
There is no rational argument that the current human population shouldn't be reduced.