Originally Posted by davrosdodebird:
“The cracks erased people's memories but the events still happened. See: duck pond scenario”
No, the events didn't happen, but subsequent events were still there.
It's not that there were ducks and no-one remembers them, but that the timeline previously contained a duck pond with ducks, but all the ducks were then removed, leaving a duck pond that has never had any ducks. (I'm not saying that that was what actually happened to the ducks, of course, that's just the analogy.)
The cracks ripped away parts of history, somehow leaving other parts mostly unaffected. It's interesting to speculate that, if the TARDIS was the source of the cracks, then the 'tear points' were more likely to be places the Doctor has had the most impact and made the timeline vulnerable - so maybe his adventures that caused the most 'upset' to the timeline were the first to be affected. It's not been said whether those have been reinstated, or if time flowed back along the 'easiest' lines.
You can't build an arched bridge without something supporting the weight of the pieces. However, once the bridge is built, the supports can be removed without everything falling down. And there would be no evidence the support was ever there.