SARS-CoV-2 isn’t really any less severe, if anything the newer variants are more aggressive, but in a way that’s a good thing. The original strain had a particularly long incubation period, meaning an infectious person could do a lot of social mixing before the first symptoms appeared, and the same for all the people they came in contact with, leading to an explosion in cases that could overwhelm healthcare systems. There were varying reports of two weeks all the way up to a month.
Now, between more aggressive strains, and pre-existing antibodies from vaccines and/or prior exposure (even if “outdated”) the immune response kicks in much faster and people know they have it much sooner and there’s less opportunities for spread (even if some assholes don’t care and mix anyway). That combined with awareness of the virus and better treatment for the manageable number of people who get seriously ill means society is able to cope much better while “returning to normality”
Still sucks for individuals who get severe cases even now.