China can't even fix its own Northeast.
Does China need to fix the Northeast though? It seems to me that the population migration is simply people moving to locations with better economic opportunities. For example, the 2020 GDP per capita of Heilongjiang was $6,236 and the GDP per capita of Beijing was $23,908. From an individual perspective if somebody in Heilongjiang found a job that payed 3-4x the salary, then why wouldn't they move? Even from a national government perspective taxing somebody making $23,908 yields more than taxing somebody making $6,236. That's not even factoring in that winters are more costly in the Northeast, than the South.
Trying to increase the population in the Northeast is pointless until wages and quality of life rise substantially or equalise with the rest of the country. Furthermore cities in the region are still increasing in population. For example, Harbin's urban population rose from 5,000,000 to 6,000,000 during 2010-2020. The total population of the Harbin prefecture decreased from 10,600,000 to 10,000,000. That means rural population is decreasing, but urban population is still rising. Whether the downsides of decreased consumer demand from a lower population outweigh the benefits of a more urban and competitive labour market remains uncertain for the region.
It's not like the Northeast is deserted either:
| Province | Area (km squared) | Population | Density |
| Heilongjiang | 454,800 | 31,850,088 | 70.03 |
| Jilin | 191,126 | 24,073,453 | 125.95 |
| Liaoning | 145,900 | 42,591,407 | 291.92 |
| Country | Area (km squared) | Population | Density |
| France | 551,695 | 65,239,000 | 118.25 |
| Spain | 505,990 | 47,450,795 | 93.77 |
