What has happened, has happened and nothing can change that.
How would we ever know? Once something was changed, everything would be changed. All memories would be changed as well...
Some things are beyond human comprehension.
Butterfly effects, unintended consequences and IF time travellers exist because time travel is possible, who is to say the travellers have the ability to actually influence what is happening in the times they visit? Perhaps they can only observe events but only negligibly affect them?
The Hitler and Holocaust scenario frequently proposed is interesting because how does anyone know that it happened and was eventually crushed because doing so thus prevented an even worse Holocaust from happening later? Endless possibilities
your assuming they would only go to the past, when they would go anywhere anytime, we would have them all through history and in the present also.
There would be no “secret”, we couldn’t keep the nuke secret, and we wouldn’t be able to keep that secret also.