they said the old game was running underneath they didn’t say they wouldn’t change anything in fact they said the opposite and did that, 28 Changes In D2: Resurrected [+More Post-Launch] for examples.
They also said they would fix some bugs and leave some in not sure where you got the keep all of the bugs thing.
yeah that is virtually impossible given the confines of 24 hour days and human needs for sleep/food/waste management.