I never said they must. But Who and How will decide which ones will not have descendants?
It is the logical consequence of your statements. If you think it is wrong of me to pave a pond where the Last So-and-so Critters live, then (logically speaking) you think these Critters (and, presumably, all other species of Critter) ought to always have descendants, forever and ever.
If that is not what you believe then perhaps you ought to re-examine your knee-jerk anti-pond-paving attitude, you see?
But Who and How will decide which ones will not have descendants?
A handful of environmentalist ideologues from Brown University who have cushy jobs working for the government, obviously. < /sarcasm >
Okay, I'll tell you. Nature decides. Or God, if you believe in him. You see, once you admit that some species will cease having descendants no matter what we do (and yes, this is a given), then it makes little sense to fetishize over every single Red-Legged Frog subspecies as if Species-No-Longer-Having-Descendants is the greatest tragedy of the universe (which it's not).
You see, the only semi-plausible argument for why it's necessary that there still be so-called "Red-Legged Frogs" in that pond even in the year 12,345 is if it is for some reason necessary for all species to always have descendants (are Red-Legged Frogs the most important species in the world? Doubtful.)
But if you agree with me, as you now seem to, that it's not absolutely necessary for all species to always have descendants... then what exactly is the argument for painstakingly bending over backwards to ensure that this particular frog species always have descendants? There is none.
Unless, of course, you belong to the Cult of the Sacred Frog which was enshrined by the Hero, Mark Twain. In other words, unless you have religioius feelings towards this frog species. Do you? Let me know,