If we take a look at the population increase from the year 1000 to the population of 2001 it looks clear that humans will over populate the Earth in just another 1000 years or so depending on how many catastrophic occurrences happen. With that I say we need to leave. Crazy.... maybe but you can't throw a paper plain to the moon. Likewise you can't make it to another Galaxy with a space shuttle. NEED A BIGGER SHIP! So we need to build one right! Fuel is not the issue we will use hydrogen from water. but who and with what shall we build it in a size that is proportionate to the needs. Particularly for the distance needed to travel and occupancy. I see the world of humans like an ant colony if we all work as a unit we can accomplish almost the impossible. Materials are not in as short supply as they may seem. Minerals com from the Earth! Under the surface is a huge supply of metals and fossil fuels. Have innumerable shuttles built to get as many people in space as we can and start construction. Design is open for debate... I can't see a ship that looks like star trek's Enterprise lol not gonna make it far.
I am not a scientist nor an engineer I just know we are in short supply of one key element, time! Just back in the 1970's the estimate of the population was 3.5 billion people on Earth at the present time 6.8 billion in just 50 years the number of people has doubled 50 more years and time will just be shorter. My view of humans is that we are a very small creature trying not to become non-existent like all the rest. I don't see us a real important. more like a small infection or something. But be us how we may I see no reason why we should not spread to other places. An ant colony moves where they must a germ moves as well. Humans are as small as that or smaller we don't have the whole picture. lol nor do I think we ever will... take the smallest known thing say like an atom or something smaller I think the Earth is small like that the Galaxy the size of a cell we are insignificant in that respect. There is no way man kind could infect the galaxy so greatly it would become nonfunctional. and if it did? so be it, it is, as it is!