Then why did the "worst" variant so far come from a place (India) with extremely low vaccination rates at the time?
It's a numbers game. With chickens there are billions of chickens created and destroyed annually, and almost all of them are vaccinated in utero, so it makes sense that we'd see the disease get selectively deadlier. With Covid if you knock down the disease rate to something that's basically a few cases per month via vaccinations, you won't have enough chances for mutations to occur.
One key difference between this coronavirus and the other endemic 4 is that mass vaccination strategies were never used. Analogous to antibiotic resistance, vaccine resistance is a realistic consequence if vaccines are used indiscriminately [2][3][4][5]. So that is one way we may be setting a new precedent here. If you take a look at what happened with Marek's disease virus in chickens, it's not pretty - none of us wants a world where it is literally impossible to survive without vaccination, because multiple generations of vaccines were undermined by viral evolution.
[1] Can we predict the limits of SARS-CoV-2 variants and their phenotypic consequences? https://www.gov.uk/government/publications/long-term-evoluti...
[2] Risk of rapid evolutionary escape from biomedical interventions targeting SARS-CoV-2 spike protein https://pubmed.ncbi.nlm.nih.gov/33909660/
[3] Why does drug resistance readily evolve but vaccine resistance does not? https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2016...
[4] The adaptive evolution of virulence: a review of theoretical predictions and empirical tests https://pubmed.ncbi.nlm.nih.gov/26302775/
[5] Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens https://journals.plos.org/plosbiology/article?id=10.1371%2Fj...