Some countries are under lockdown, with healthcare systems collapsing, or such widespread uncontrollable transmission that simply leaving the house would be dangerous. I fully understand why players in those countries would want recycled legendary raids and to be able to play completely from home.
On the other hand, other countries (including mine) have close to zero community transmission (only imported cases) and robust testing/tracing capability. In such countries, going out is good for health and poses minimal risk of infection, so local measures do not prevent normal gameplay. Players in such countries would want highly engaging events and meeting this demand would earn Niantic more money. The only bonuses they need are QoL improvements such as guaranteed gifts from stops and increased interaction radius.
Of course, most places are somewhere in between, ranging from “several clusters but still under control” to “large community outbreaks but nowhere near emergency/lockdown levels yet, restrictions make normal gameplay still possible but reduced”. These countries would benefit from bonuses such as half distance for eggs (and Adventure Sync).
The key advantage of a tier system for countries, with different bonuses for each tier, would be flexibility in tailoring bonuses to the local situation and easily adjusting bonuses by moving a country to a different tier should its situation change. The hard part would be designing the criteria for each tier and evaluating countries against each tier, but perhaps external agencies (such as the WHO) may have already done so?
Of course, Niantic are unlikely to be competent enough to pull this off successfully, but they have shown some sensitivity to the different situations in each country (for example, by cancelling events only in some countries back in February/March and choosing three cities with low levels of transmission for City Spotlight).