It depends on which titles. They typically remain in print for half a century and more (they start contemplating a new edition after 40 years in print, normally), and most titles remain available over a decade after their last printing.

Abandoning titles is the exception rather than the norm. It's happened to more minor authors that they probably should have waited longer before they included them in the first place and that have totally fallen out of fashion and don't sell.

They've totally abandoned L'Encyclopédie collection as it proved no longer viable financially, but most of the works have resurfaced in Folio.

There was a good interview with Gallimard folks about the collection a few years ago. The collection is a huge challenge for them. They need big printings to be able to sell them for a reasonable price, but it means their stocks for most titles last for years, sometime over a decade, and storage to ensure these books remain in pristine condition for many years is very expensive. Bookstores typically store only 25 titles or so (the newest additions, a few of the best-selling ones), and with the spread of internet bookstores they don't store much of anything except the new ones and get the rest from Gallimard on order. The more the collection grows, the more an investment it becomes, especially since they're hardly stored in normal conditions since they need to remain in pristine shape for years.

They're not about to abandon it, though. On the contrary, Gallimard is convinced if physical books are to have any long term and widespread future it will be with such upper scale editions (French publishing is currently in serious denial about ebooks).

I know for sure you do own a few titles which are OOP, though. The medieval chroniclers, for example. That's from their old medieval collection. Renart, CdT etc. are from their new one.