That explains why the organism is still around - our theory fits the observation. it still does not explain what the organism gains by reproducing, and the gain of an advantage in the environment is the key to adaptation.
A type of organism that cannot reproduce cannot spread. That means there will only be one organism of that type on earth. Any day that organism could be destroyed by being crushed in a landside, eaten by something else, or falling to it's death. Once that happens that type of organism no longer exists.
On the otherhand a type that can reproduce will spread many copies of itself all over the place. Now accidents and predation might destroy a few copies, but the chance of every organism of that type being wiped out is very small due to the ability to reproduce. That is the advantage.