That is true! Maybe the disease burden increases when new pathogens are introduced but eventually reaches a balance.
But it doesn’t seem to completely remove the effect from accumulating multiple pathogens in the community, a population with covid+flu (or any other combination of pathogens with no cross-immunity) in circulation and adapted to live with them will probably still have a higher disease burden than a population with just the flu in circulation and adapted with them.
Quite likely that the 19th century was worse than before due to increased population density and global connectivity, and I shudder to imagine what if AIDS became established in humans in the 19th century instead of the 20th.
I think the second reason “against” is probably the only true argument that my hypothesis is wrong.
The first reason still doesn’t prevent the disease burden after covid/any new pathogen spilled into humans from being appreciably bigger than the disease burden before the event.
The third reason is of course about interventions, which can go...many ways.
I didn’t raise it in my original question but I wondered if this hypothesis applied to non-human species, which would still be a pretty interesting problem since it might impose a limit to the propagation of any species (as it collects more different kinds of pathogens overtime and need to contend with them).