A few notes, touching on various elements involving preservation:
1) I'm not sure that the missing late NBC programs mean anything with NBC- in addition to the points raised concerning High Rollers and Dream House (though the version I've heard for the latter involves flooding), Sale of the Century seems to be a Reg Grundy issue, as Going For Gold (which aired in the United Kingdom into the 1990s) is similarly missing a pile of episodes.
2) I would be cautious about making assumptions about networks in terms of preservation, based on examples from other genres:
2a) There seems to be an idea that CBS stopped wiping earlier than the other networks due to the survival of the Goodson-Todman programs and the rediscovery of The Joker's Wild. The notes that soap opera fans have compiled concerning what survives in that genre make this suspect- the CBS soap operas of the 1970s do not seem to be particularly well-preserved, further suggesting that it was based on production firms;
2b) That said, even production firms could be inconsistent- there is a general consensus among those who have researched television sports broadcasting that ABC seems to have done a better job of preserving their sports broadcasts from the 1960s and early 1970s than either CBS or NBC, but there are a number of places where gaps are apparent, with their coverage of the NBA from 1964-1965 to 1972-1973 existing (on the basis of the available evidence) in a fragmentary state indeed (total number of NBA Finals games known to exist completely on master-quality tape from this period: one).
Overall, this ties back to considering the survival of the programs in a key regard: while we have some ideas of what is out there, there are still limitations to our information (note that, twenty-five or so years later, we still aren't sure of the total contents of that Hollywood Squares rediscovery, to name just one example) that are such that I would be deeply wary about assuming any general rules existed, rather than a collection of individual circumstances for which consistency cannot be assumed.