True, with our present knowledge and technology it is not possible. That does not mean this will always be the case.
Anything is imaginable, but I just don't see how we can realistically expect to ever build the conditions to make true space travel feasible. Some limitations are direct consequences of the very nature of the universe and natural laws.
We will have a hard enough time managing the levels of population and ecological waste. Manned space travel is a silly mirage IMO.
The distance and economics of landing humans on the moon was once considered insurmountable.
And it arguably was indeed. The expense was astounding, and that was with the moon attracting the vessels half the time. It shouldn't be taken as representative of space travel at all, for it is quite the anomaly, a cheat even.
Also, what is a "nest" proposal?
The idea of launching just enough people and resources into space to try and rebuild society from scrath with the bare minimum people.
It is a last resource strategy for avoiding the full extinctin of humanity. A fascinating sci-fi scenario, but even that is quite definitely beyond actual possibility.