The problem with these discussions is everyone seems to assume colonising a galaxy is just like colonising a planet - so you have an Age of Exploration, and then you set up colonies, and stuff happens, and you have an Empire or a Federation, or something.
This is really, really unlikely to be true, for all kinds of reasons.
There's a huge difference between colonising a galaxy - which assumes some kind of uniform culture, and common strategic goals - and simply throwing more or less living/sentient stuff around a galaxy and hoping some of it sticks.
We've gone from barely being noticeable on Earth to being in danger of being responsible for a planetary extinction event in around 12,000 years.
We know nothing about building a stable culture that can last for millions of years. It's unlikely any sentient species will be able to learn that skill without making a lot of mistakes - some of which will be terminal.
So galactic colonisation requires a lot more than solving the mechanics of transportation. And the "few million years at 0.1c" estimate is going to be Not Even Wrong.
You don't need to build a stable culture for a galaxy wide colonisation.
We colonized the whole planet without that. Likewise a locust-like swarm of colonizing ships without any central control or common culture would also be a possibility.
This is really, really unlikely to be true, for all kinds of reasons.
There's a huge difference between colonising a galaxy - which assumes some kind of uniform culture, and common strategic goals - and simply throwing more or less living/sentient stuff around a galaxy and hoping some of it sticks.
We've gone from barely being noticeable on Earth to being in danger of being responsible for a planetary extinction event in around 12,000 years.
We know nothing about building a stable culture that can last for millions of years. It's unlikely any sentient species will be able to learn that skill without making a lot of mistakes - some of which will be terminal.
So galactic colonisation requires a lot more than solving the mechanics of transportation. And the "few million years at 0.1c" estimate is going to be Not Even Wrong.