Old age is no rational reason to dismiss a book.
Even though Hilberg's "Destruction" has enjoyed updates and revisions, the latest in 2003, the first edition of 1961 was already excellent and would deserve being quoted and used today.
Biologists often quote Darwin's "Origin", 151 after its first release, because it was an excellent book with lots of good, original concepts. I might even go back to Aristotle's Organon - not forgotten because it is, well, a good body of work.
Read the thread I linked to. Butz's book can be dismissed in relation to what he should have known or been able to find out
at the time, in the early 1970s. He strawmanned the state of knowledge back then.
But since 1975 there has been a massive volume of research on all aspects of the Holocaust. "Outdated" is a frequently heard term in historiography and in science, because historiography and any other form of science expand exponentially due to new research. There may well be books that survive because of their literary merits, or 'wear well despite their age', or can offer a particular insight.
Hilberg's book is one of them, but in relation to the state of the art, it's pretty outdated now. I'd recommend as overviews, Friedlander, Longerich, Bloxham (as a short and provocative intro essay) and then Hilberg. The expansion of research means there is less and less reason to cite Hilberg since there are more recent and more comprehensive books on different regions, countries, camps, units, etc. This doesn't detract from Hilberg's achievement. I think he'll be read for some time to come, even though his work does not have the literary merit of, say, Huizinga or Burkhardt, who are still read long after we know
infinitely more about the eras they portrayed.
Unlike Hilberg, Butz did not revise and expand his book, he simply bolted on a few essays to a 2003 edition. The main text is the same as it was exactly 35 years ago in August 1975 when he wrote the preface, and boy does it show. The book reads like it's from another era, and the cobwebs are visible. Many of the arguments he makes are no longer made by deniers, who have in some cases publicly rejected them. So it's not even a very good introduction to revisionism, and fares worse than Hilberg does in comparison.
Deniers have tried to write overviews of their theories since Butz, but they are all worse than even Butz managed. Butz claims in the preface to the 2003 edition that his book was still the only revisionist volume that told 'the whole story' - yet the book left out most of the story!
It's still telling, as I said, that deniers tout this 35 year old p.o.s today.