In his essay "The Unnatural History of Tolkien's Orcs", the author "Tyellas" references the same passage from "Morgoth's Ring" (without quoting it or giving an accurate bibliographic reference) that states Orks are not immortal. He also makes good arguments, with similar references, for their "creation" as corrupted Elves.
As stated in this conversation, I am also of the mind that while Morgoth could corrupt Elves, he could not deprive them of their nature imbued by Eru. Elves are immortal and can choose to return to Arda. After they "die", they go to the Halls of Mandos to wait to return or until the end of the world, because they are tied directly to the fate of the world.
In corrupting them, I think Morgoth tried to deny them the choice of how or when to return, and he may have denied them the ability to coexist in the spiritual and physical worlds. Besides corrupting them to evil, he could have bound them to an endless existence of a harsh life, a harsh death, and the inevitability of doing it all over again. Kind of like the Tom Cruise movie "Edge of Tomorrow" or Michael Morcock's concept of the Eternal Warrior.
In the aforementioned essay, the author also makes a case for the potential of Orks choosing to become uncorrupted, which is in keeping with the idea that Elves can choose how they evolve throughout their immortal existence. Reducing them to base instincts ensures there is always some Ork somewhere looking for an opportunity to breed, which in turn provides an opportunity for a fallen Ork to return - no extinction!
Daniel "Vaco" Vacaflores' article "Quendi" in issue 12 of "Other Minds" magazine (othermindsmagazine.com) expounds on the nature of the various types of Elves based on analyses of professor Tolkien's works. In the article he discusses the consequences of their immortality and their ability to choose how they personally evolve through the ages, which provides the basis for the idea that Orks could potentially, though perhaps highly improbably, choose to become uncorrupted.