Aside from standard cataclysms like comets and supervolcanoes wiping out most life, the Sun is a timer as well. There’s definitely less than a billion years until the Sun boils away the oceans as it heats up. Obviously outside of any timescale any living thing could comprehend, but would you like to gamble on evolution producing another intelligent, spacefaring species if it has to start over in irreversibly deteriorating conditions, assuming we don’t make it? It’s now or never - we must expand beyond Earth because each moment we delay, the chance of extinction only increases.