Well, if people live for really long time like 10000, it would become much easier to travel to other stars with technology that we already have, so there will be plenty of "ecological niche to live in".
Would it really be much easier? It's already possible, we just would have new generations on the ship when we arrive. We don't care enough about those future generations to take off for a new world today. Will we care more about our own 10000 year futures?
Passing down skills and ideas needed for the mission to survive and succeed over multiple generations is a very hard task.
A group of skilled and motivated people who spend a small percentage of their lives on a ship, is going to be very different from a group that is trapped in a small town for generations.
My estimate would be that the mission to succeed is going to need 10-100x more people on generational ship compared to a transport ship. (million vs tens of thousands.)