I still tend to think that this can most easily be explained by the fact that India hadn't been hit hard up until now and that a lot of people started to believe magical theories about how they were immune, and now we're just seeing the virus ripping through high population density cities and overwhelming the health care system.

If you want to talk about variants, though, then there's a couple of things to consider. The first is that the "avirulence" theory that pathogens evolve to become less virulent over time was pretty decisively falsified in the 90s, so forget everything you think you know about that idea being true. There is a fairly well-studied tradeoff which has replaced that between transmission, virulence and in-host viral replication. That doesn't suggest the direction of evolution though. However, there have been studies of incomplete vaccination and immunity which suggests that it actually works to increase selection pressure for higher transmission variants and for virulence. This intuitively makes sense since as, say, half the population has immunity that increases the pressure on the virus to increase its fundamental transmissibility since an R0 of 3.0 will no longer be anywhere near as effective -- and the mechanism to do so is to just increase viral load and thereby increase virulence and kill more infected.

And this may have been observed in 1918 with the more lethal later waves of the pandemic which affected younger people. That virus later evolved into seasonal influenza, probably as a result of widespread immunity producing pressure for immune escape variants which came at an overall cost to fitness by mutating all the epitopes on the surface proteins so that it could re-infect already immune individuals. That will probably happen here as well, there'll be a limit to how virulent and transmissibile this virus can get and it'll have to start making costlier immune escaping mutations eventually -- but in the short term I wouldn't be too surprised if we start to see a lot of dead younger people in third world nations that don't have as much access to vaccines going forward.

But at any rate, none of this particularly matters. The lesson is still all the same which is to lockdown and avoid catching and spreading the virus until you have a chance to get vaccinated. Which is what it has been all along. People still try to argue with the virus and engage in magical thinking about how certain populations have magical immunity to it, but that continues to keep getting falsified. India most likely just got lucky that they didn't get hit early, then the prior wave they had was their "seeding" wave which was likely just tamped down by people voluntarily taking precautions. Then based on their success everyone dropped their guard and thought it would all be over in 2021 and the virus just took advantage of that.

People in the US who are vaccine-hesitant should also consider that if they've managed to not catch it so far and develop any immunity that you really want to avoid rolling the dice with later versions of the virus. Also it should be noted that while the virus kills a lot more people over 80 that the doubling rate of fatalities is every 8 years of age while the doubling rate of hospitalization is only ever 15 years -- so a 40 year old person is 32 times less likely to die than an 80 year old, but they're only 6 times less likely to be hospitalized (although it falls off a cliff once people get under-20 and the very young are much less likely to be hospitalized). The way that American culture is so focused on our mortality to the exclusion of all other outcomes means that we're heavily discounting the possibility that we could survive the virus but be physically crippled by it.

> "Virulence evolution and the trade-off hypothesis: history, current state of affairs and the future."
> https://onlinelibrary.wiley.com/doi/full/10.1111/j.1420-9101...

> "The Origin and Virulence of the 1918 “Spanish” Influenza Virus"
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720273/