I don’t know of any plausible axiological theory on which this is false. On the total view, an increased population is good as long as each person’s life is good, and they aren’t such a burden on everyone else so as to drag down the overall welfare. That seems quite obviously true in the modern world. On the average view, depopulation is bad as long as it lowers average welfare, which it almost certainly will due to the inability to run institutions built in the higher-population age, the loss of economies of scale, etc. Even on a person-affecting view, depopulation is bad—it doesn’t just negatively affect people who never come to exist as a result, but also the people who do exist.
I don’t know of any plausible axiological theory on which this is false. On the total view, an increased population is good as long as each person’s life is good, and they aren’t such a burden on everyone else so as to drag down the overall welfare. That seems quite obviously true in the modern world. On the average view, depopulation is bad as long as it lowers average welfare, which it almost certainly will due to the inability to run institutions built in the higher-population age, the loss of economies of scale, etc. Even on a person-affecting view, depopulation is bad—it doesn’t just negatively affect people who never come to exist as a result, but also the people who do exist.