Why would it be MOST important for the Party to be sure that,
the past had not merely been altered, it had actually been destroyed
as according to Winston?
The past is just that… the past. There is nothing worth remembering from it.
It’s important for the people to have no memories of anything from the past that may prove harmful to the Party and their reputations.
There's no reason to have any recollection of the past since the citizensn are living in the present.
The changes from the past were incorporated in the best interest of the citizens; the Party wants to continue to keep them safe.
The Party does not want any adversity brought up against them and if the past is something that is remembered, there could be actions that pose as a threat to the Party and causing the citizens to realize that they have been lied to brain-washed to believe falsified statements.