Every Possibility

Every possibility: that phrase applies both to the hypothetical multiverse and to Poul Anderson's works. A pessimistic ending to a particular work, e.g., "The House of Sorrows," need not imply that the author himself was pessimistic or indeed that he adopted any single attitude toward future possibilities. In The Shield of Time, the possibilities include an endless cycle of rising and falling empires never relieved by any scientific or industrial revolutions but what the Patrol instead preserves is a history that:

"...does at last take us beyond what our animal selves could have imagined."

-Poul Anderson, The Shield of Time (New York, 1991), PART SIX, 1990 A. D., p. 435.

While the Danellian explains, the wind of course comments although with an underlying note of menace:

"The wind cried, the sea growled nearer." (p. 434)

(In the Bible, the sea is the pre-cosmic chaos always threatening to return.)

"The Voortrekkers" also has an unexpected utopian ending. See here. 

Anderson's two most basic values are diversity and freedom:

"'...we've always had variety, always had the rebel and the heretic. We need them!'"

-Poul Anderson, Planet Of No Return (London, 1971), Chapter 18, p. 126.

"'Men ought to be free.'"

-ibid., p. 127.

And this might be more practicable on an interstellar scale:

"'And if things go wrong somewhere in the Galaxy, there may well be other places where they go right, more right than you or anyone else could predict.'"

-ibid., p. 126.