Aw yeah, I hope you don't mind this absolute essay I'm about post here, OP, but I've spent way too much time thinking about this.
The Wizarding World is a society on the brink of collapse.
Now, I don't think JKR intended this or even really thought any of her choices through beyond 'hurr durr magical nazis, Voldemort's a half-blood just like the antisemitic and debunked rumor that Hitler was also Jewish!' (and fuck her anyway).
The Wizarding World cut off contact from the Muggle World with the Statute of Secrecy in the 1600s. Since then what few Muggle technology that has managed to penetrate this barrier are few and arrive very late. The latest Muggle tech that has transferred over is the train and the radio. No television, no vacuum cleaners, no cars (except Arthur's mad experiments in the shed). Their fashion (in the books at least) has also stagnated, such as the fact that Arthur and Molly are wearing Muggle clothes in GoF to blend in but Harry notes they didn't do a very good job. (regarding SWM, Snape isn't wearing trousers either and I don't think that any of the students are except for maybe the odd Muggleborn who would feel too weird to go without; the movie uniforms have become so ubiquitous that this scene can be a little confusing.)
I say all of this because as the 1940s rolls around, the Wizarding World is being suddenly and violently yanked into the Muggle World.
I 100% agree with you that Tom Riddle doesn't actually give a shit about blood purity, it's just a tool he uses to gain what he really wants: power. The Death Eaters are what's known as a cult of personality; Trump is an excellent example of this. Trump could strangle a puppy on stage and his most die-hard followers would be like, "It needed to be done. He was sent my God to make America into a Christian country again. That puppy was a transgender pagan communist." But Tom Riddle also grew up in London in the 1940s. He would have seen the Blitz first hand, he might have even been evacuated like other London children. His pureblood contemporaries would also be experiencing the same thing.
The shortages of food, cloth, and other goods would have affected the Wizarding World as well. Diagon Alley might have very well experienced some damage to the Blitz; while the magic might have prevented an airplane from seeing Diagon Alley, it wouldn't have protected it from being carpet-bombed. And, most importantly, I don't believe there is any magic that could have protected wizards and witches from the atomic bomb.
That would have been an absolutely terrifying thing for the Wizarding World to come face to face with. For so long they've viewed Muggles as little more than animals that can be obliterated and stolen from with ease. Now Muggles had the ability to wipe out entire cities from a distance.
And afterwards, as the Cold War starts to brew, Muggleborn students are entering Hogwarts with books like this one: Protect and Survive | Wilson Center Digital Archive, a pamphlet telling British families how to react in case of a nuclear attack. Muggle technology is leaping forwards as computers are being developed, household appliances, radios are becoming smaller and portable. Oh, hold on, wait, what's this? Muggleborn girls are wearing minidresses? Bellbottoms? Hippies? Disco? Rock 'n roll? A cultural revolution is underway and Muggleborns are taking their own culture and social mores with them as they enter the Wizarding World.
Now, whenever there is great social upheaval, there are usually cults. There were a ton of cults popping up in the 1960s and 1970s, such as the Manson cult, the Rajneesh Movement, the Children of God, etc. The American Christian "Great Awakenings" also coincide with big societal changes (1790s- the Revolutionary War, 1850s- the Civil War, 1930s- the Great Depression, 1960s- the Swinging Sixties). It's human nature to want to find something to cling to as a life raft during turbulent times. The First Wizarding World started in 1971, and Voldemort had probably started his cult before then, working in the shadows. When he arrived on the scene, the Wizarding World was primed to be taken in by a cult. The dominos were lining up perfectly.
Now we have the Second Wizarding War. By this time, Muggle fashion is becoming commonplace for Harry's generation. Even a young pureblood Draco Malfoy probably wouldn't so much as bat an eye at a woman in a short skirt and go-go boots except to think she was horribly out of fashion and needed to get with the (Muggle) times. Technology still isn't making much headway into the Wizarding World, but since electricity and magic don't play well with each other and this is still the 90s when a cellphone looked like a gray brick with an antenna attached, that's not surprising. Despite some comments from Draco and his ilk, Harry's generation is relatively peaceful and stable. In the Muggle world, the Iron Curtain has fallen as has the Soviet Union. The Cold War is over. The economy is up, up, up.
To understand why Draco's generation would join Voldemort's cult we have to understand that this is something they were raised on. Because it is a cult, a religion, and Voldemort told his followers that he could not die, that he would return, and these kids are learning this at their parents' feet. They are not nazis, they are a radical Christian cult that are trying to trigger the end times and bring about Rapture where they -- the chosen few -- would be yoinked into heaven while the undesirables suffer for their sins on Earth and in Hell. Voldemort here is Jesus, he is King Arthur who will one day return during Britian's darkest days. That, my friends, is a fucking cult.
And make no mistake, almost anyone can get sucked into a cult. They are absolutely insidious, and the techniques they use qualify as brainwashing. It doesn't matter how smart you are, in fact some studies show the smarter you the more likely you are to stay in a cult (it's that cost-sunk fallacy, baby). I've seen plenty of TikTok trends that have gone viral that make me think, hm, this is a little culty (tradwives, I'm looking at you). You are not immune, and neither am I.
(Also, as a sidenote, the Wizarding World is absolutely fucked. The stories end in the 1990s, but now we're in the 2020s. There are recording devices everywhere, there's even one in your pocket! Sure a wizard obliterated that memory of you seeing magic, but will he know how to delete the video from your cloud?)
