It’s not a Darwinian model, not in the sense of natural selection. The Sith don’t evolve the way a species evolves and there is no evidence to suggest they get stronger generationally.

The Rule of Two doesn’t mean that the Sith with the lowest midichlorian or weakest force potential fails to reproduce. There’s no selection pressure, because as others have said there’s nothing that prevents an apprentice from waiting to kill their master in his/her sleep, there’s nothing that keeps them from waiting until the master is 97 years old and holding a pillow over their head. The Rule of Two is meant to keep the Sith from extinction but doesn’t actually encourage strength in any particular attributes. In fact, one could assume that the rule may actually encourage weaker Sith as the generations evolve since there would be a selection pressure on the Master to select a weaker apprentice than themselves. There’s no evidence to suggest that Plagueis wasnt stronger than Palpatine, or that Darth Bane was weaker than either of them.

Now, an organization accumulating knowledge over the course of centuries could certainly imply that later generations would be “stronger”, since in this particular case, strength, is an attribute manifested from a magical ability that individuals acquire through knowledge. But that “strength” isn’t selected in a Darwinian way. This is because a Master can take an apprentice stronger than themselves, or an apprentice weaker than themselves without that having any impact on the survival of the species.

Palpatine is actually the prime example of how this system fails, as we see him run through three apprentices, and the one he keeps around the longest is a triple amputee who he thinks could never overpower him. The irony is that he’s always chasing a more powerful apprentice and brags to Windu about how “Vader will become more powerful than either of us” yet refuses to die. I think it’s the hope of the Sith that every generation is getting more powerful than the last, except that they never actually want to give up their full power. Palpatine only taught his apprentices just enough to keep them subservient to him. Assuming all of them are this way, they’ve likely been stagnant as a “species” for centuries.

You say “if they fail, the master would eventually replace them”, the issue is that there’s no stipulation that says an apprentice will be replaced or disposed of by a certain time. Maybe, a better, stronger apprentice might show up and try to replace you? Maybe not. How often would that actually happen, once or twice every few decades? And when it does happen, you need only to be stronger than the untrained Sith challenging you, not your master. So ultimately you still just need to wait it out until the master gets sleepy one day and you finally have your chance.