The fundamental issue with declining populations - fundamental as in regardless of the economic system of the country - is decreasing standard of living.
The very simple metric is productivity-adjusted hours worked per person. This invariably falls in cases where overall population is declining, because populations age as they decline, and older people work less (retirement) than younger ones.
As this metric falls, the country’s economy basically just produces less stuff per-person than it did in the past. This makes everyone effectively poorer.
In extreme cases, there can also be issues with availability of services. E.g. healthcare: Each doctor/nurse/caregiver can only effectively attend to so many patients and this number is difficult to increase with technology.
Those without respiratory issues don’t have oxygenation issues while wearing masks.
The same air is going into your lungs, the only difference is your diaphragm has to work harder due to the filtering effect of the mask. If it can’t manage that, then you are likely already on oxygen due to low tidal volume and chronic hypoxia.
You can buy a pulse oximeter from CVS for like $20 and test this yourself if you don’t believe me.