For the Spanish flu, there wasn't the same population density, sharing of overcrowded public transport, the sheer movement of people around the planet on planes, and general propensity to get up close to people regardless of what other people wanted. (Yes, there were apartments and tenements, and public transport was used - I am talking about the quantum of overcrowding that went on).
Those countries which locked down hard had lower levels of incidence, severity and mortality.
Those countries which were more lax saw higher numbers infected, more seriously ill and dying.
And the lockdowns were as much about ensuring the public health systems globally, many of which were heavily over-stretched, did not utterly collapse under the sheer numbers of sick people needing treatment.
Yes it has caused many short and longer term problems for individual countries and globally, in a wide variety of ways.
But think how much worse it would have been - sending the kids to school but not knowing if a teacher would turn up because they were sick (or too scared to). Going into the office but the security guards and cleaners being sick so it's filthy, if you can even get in the door. Senior managers having to go to work but getting sick and not being able to manage the big systems of government and public services. People insisting on going out in their cars because "it's only flu" but passing it on even more.
By having clear strict rules, people could get the essentials and essential systems stayed online. Lots of people were supported to not work in the public interest. Many people learned the new skills of remote working, and the business of government, and many commercial businesses, stayed operating with lower levels of absence due to illness because of remote working not passing on the virus. There were fewer dangers for going out to work for those essential workers who did need to turn up in person (healthcare, electricity and broadband and water services, bin men, shop workers etc).
I have personally had a lot to deal with in fall out from lock down, and I have been incredibly lucky in many ways. But I still would have done it, and would do it again if it was necessary.
Those who locked down early like Tonga and Fiji had very very few cases overall. Even New Zealand and Australia.
I was tracking case numbers in the early days (the first 3 months) and graphing the quantum of changes. Ireland had and earlier and a far stricter lockdown, and adherence to it, than the UK - the numbers of those ill initially were relatively similar proportionally to population size, but in the UL they climbed exponentially whereas in Ireland it was a far more steady line of increase and it curved back down again much quicker. Both in terms of infections, numbers in hospitals and also numbers of deaths. Literally the like-for-like daily numbers.