Earth's demise and the need to colonize other planets

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Ok, so we know by now that some rare events can wipe out a good portion of life on earth; Asteroid/Comet, Super Volcanos, (Galactic Gamma Burst?), etc. We also beleive the Sun will expand in the far future, scorching the surface and blowing away what's left of the atmosphere. Global warming could eventually cause a lot of damage, floods, famines, desertification and other event could reduce our ability to build a space colonization infrastructure. But there might be something else, that unlike Asteroid impact might be certain to happen, and unlike GHG global warming could not be fixed by changing policies... ...The cooling/solidification of earth core, or if you prefer the demise of earth's magnetic field. Unlike the sun's expansion which is unlikey to occur before millions of years, the immobilization of earth rotating core(or the depletion of fissionable uranium, depending on the theory) could occur, sooner than we think. When the magnetic field fades, we'll be seeing auroras in the sky and the earth will be receiving much more radiation. I dont know how plant life will cope with the increase in direct radiation in daytime. Should this come to pass before humans colonize on an other planet, it may be difficult to expand into space for some time and we'll need to colonize earth since it will no longer be as fertile as it once was (but lacking colonies and the derived technology will make it more difficult, hence fewer people will make it). Another aspect of an earth without a magnetic field is the erosion so to speak of the atmosphere from the solar wind. Earth might slowly loose some of its atmoshere over a long period of time. There are traces that Mars once had seas and rivers(perhaps thicker atmosphere as well?) and that it once had a magnetic field similar to earth, but it lost its magnetic field when the core cooled and later on appears to have lost some(or most?) of its water. Has anyone heard of the nuclear fission theory, according to which planets had uranium in their core generating energy through nuclear fission, but that once the uranium is depleted the core cools down much faster, smaller planets cooling faster than larger ones? There are probably people here who know a lot more about the planetary magnetic field generation and depletion than I(since I hardly know anything about it), so I'd like to read your comments about this (and about colonization of other planets in general).