Future History is admirably compact, adequately covering the several successive periods of its fictitious history in just four volumes. Orphans Of The Sky is a short appendix about the crew of a single spaceship that does not advance the history of mankind.
We need a complete uniform edition of Poul Anderson's Psychotechnic History, which would probably fit into five volumes of normal length or two omnibus collections on the model of Baen Books' Technic Civilization Saga.
Readers of science fiction should have the opportunity to read right through the Future History, the Psychotechnic History and the Technic History, beginning with Heinlein's "Life-Line" and ending with Anderson's "Starfog." Whereas the Future History is compact, the Technic History is necessarily long both because it covers more space and time and because it incorporates entire series that had had a life of their own before they were connected into the History.
Larry Niven's and Jerry Pournelle's future histories succeed Anderson's but, for me, the peak of future historical fiction remains the Technic History.