They are all very sad about this. But the series isn't over yet.
Star Rangers (aka 'The Last Planet') by Andre Norton is another story about finding a long-forgotten Earth. In this case, however, the weary crew of a spaceship -- the spaceship being a poorly-maintained relic of a dying galactic civilization -- crash lands on the planet by accident, and for most of the book neither they nor the reader knows that this accidental discovery is the original home of the species.
(There are some nice hints along the way. The green of the vegetation here seems somehow the best and the purest green, unlike the blue-greens or the yellow-greens of other planets they've known.)
The moment in which they find out where they really are ("Terra of Sol - man's beginning!") is a moment of sheer wonder and science fiction glory. Earth had been abandoned by (most of) its people, who left to find the stars. The crew members have a vague memory of this abandonment - it has been a story handed down over the long generations - and now they see the ancient 'Hall of Leave-Taking' for themselves. Kartr - our protagonist - thinks to himself:
"There --why, right there had sat the commanders, and behind them crews and colonists! And so they must have gathered, shipful after shipful for years - maybe centuries. Gathered, spoke together for the last time, received their last orders and instructions - then went out to the field and the waiting ships and blasted off into the unknown - never to return."
Wow. Of course, my personal fantasy at the time (early 1960s!) was to leave Earth myself, permanently. ASAP. So, together with a strong theme element of telepathy (very cool) the book was this adolescent's dream.
I will mention one slightly iffy plot point; shortly after our hero crew crashes on the planet they discover another spaceship has crashed there a few months before. Then, about the time they realize what planet this is, another spaceship lands (and is soon destroyed.) Although the various passengers on these ships are necessary to the story, it does seem a little funny that - after Earth has been (nearly) deserted for millenia - it is suddenly beseiged with crashing spaceships.
Did you notice the 'nearly' deserted bit? Yes, there are still people on the planet. They have retreated to a pre-technological society, and the author describes them somewhat like pre-colonial tribes of native Americans. She does not condescend to them however, and one of our heroes remarks that the people who did not leave Earth - and who have lived there ever since - simply made the choice that seemed best to them at the time.
And indeed, it is the choice that our heroes make as well. Rather than attempt to live in one of the old cities, with some technological comforts, Kartr and his crew decide to strike out and live off the land as newcomers. The end of the old galactic civilization is the beginning, for them, of something new.
