Although Eldritch Wizardry was released in 1976, the copy I have is from the 9th Printing in November 1979, which isn't surprising, since I didn't get into the hobby until Christmas of the same year. Consider that date for a moment. The Dungeon Masters Guide, the final volume of AD&D was first released in August 1979. What that means is that TSR was still printing OD&D books several months after the completion of Gary Gygax's magnum opus.
As it turns out, TSR did reprintings of all the OD&D books, including the White Box (now called the "Original Collector's Edition") in November 1979. That's actually quite interesting to me, because it gives some credence to Gygax's claim that AD&D was "a new game" and that TSR intended to keep OD&D on the market for the hobbyist market. Of course, that's not quite what happened in the end, with OD&D morphing into the more mass market-friendly boxed D&D lines (first Moldvay/Cook and then Mentzer). Similarly interesting is the fact that TSR's last printing of 1st edition AD&D Players Handbooks was in July 1990, which is over a year after the premier of 2nd Edition.
What this suggests to me is that there was clearly still a market both for OD&D and 1e after their successor products were published. Otherwise, TSR would not have bothered with new printings at all. This further suggests that the biggest impetus for both 1e and 2e came from TSR itself, not the existing players of the game or indeed newcomers to it. I suppose that shouldn't be surprising, but it's a little disappointing to see clearly that the inevitable cycle of new editions began very early in D&D's existence and could, in many ways, be called one of the game's oldest "traditions."