But they have existed for a short period of their lifespan, so by definition are early in their lifetimes. The argument isn’t that there will be new stars, but that they will exist for a long time.

There's good reason to think life is not going to arise on planets around M stars. Our sun is quite bright compared to the typical star, is already gone through about half its time on the main sequence, and as the Sun continues to brighten the Earth will become uninhabitable in about 1 billion more years.

Moreover, life originated early on Earth. This suggests either that more than one extremely rare event is needed to get from life to us, or that the conditions for OoL do not persist for very long (ammonia, for example, is rapidly destroyed by solar radiation.)