And that's all well and good but it's little more than a theory, it also assumes other species would even want to travel to the stars. For an example of what i mean, the Ancient greeks came close to inventing the steam engine http://en.wikipedia....f_Alexandria ina time when the great library of alexandria was still standing. You can imagine what the world would like now if they had. Technology isn't something that you just put your mind to and presto, it's about investing time and resources in the right places, in the right moment and if the time and resources aren't invested? You don't invent the steam engine for another 2000 years or even at all. That's not even taking into account that they might simply not be interested, iirc the only reason we did it was because two super powers were having d*ck swinging competition.
First of all, its not a theory. It's not even a hypothesis, really. Sorry, pet peeve of mine is people not knowing what these words mean. Secondly, you missed the point entirely. The point is, it only takes ONE species achieving this just once to completely colonize the entire galaxy in a cosmic blink of an eye. That's what is so impressive by it, and that's why the Fermi Paradox still stands as a legit question. If you don't come away from it absolutely awed by how comparatively quick a species can colonize a galaxy with sublight travel, then you dont understand what Fermi was talking about.
Again, if WE wanted to do it, which we do, we could colonize every star system in only 50 million years. And our sun and our planet are relative newcomers in the galaxy. There are planets billions of years older than earth.