TLDR: the polio vaccine used to contain weakened versions of the three strains of poliovirus. When weakened live virus vaccines are used, the people inoculated with them shed copies of those viruses, which is usually no big deal… except that one of those weakened polio strains would, very rarely, mutate back into its full-strength form and sicken unvaccinated people living around those who were being vaccinated.
Eight years ago, the decision was made to remove the problematic strain of polio from the vaccine, because it was thought low wild infection rates meant that the risk of vaccination-derived infection had become higher than catching it from the environment. Regrettably, it seems that decision was made in error – type 2 polio outbreaks have soared since then.
I agree, this is a good use of the live service model to improve the gameplay experience. Previous entries in the Flight Simulator series did have people purchase and download static map data for selected regions, and it was a real pain in the butt – and expensive, too. Even with FS2020 there is a burgeoning market for airport and scenery packs that have more detail and verisimilitude than Asobo’s (admittedly still pretty good) approach of augmenting aerial and satellite imagery with AI can provide.
Bottom line, though, simulator hobbyists have a much different sense of what kind of costs are reasonable for their games. If you’re already several grand deep on your sim rig, a couple hundred for more RAM or a few bucks a month for scenery updates isn’t any big deal to you.