Existential Risk includes the edge-case that we might damage civilization so badly that we're trapped on this planet until the sun goes supernova, yes. Or become nothing more than pets to some AI. But it's a pretty niche edge case - if we meaningfully continue as a species, expanding out into space seems pretty inevitable. And, honestly, I don't think anyone really views the edge case of "we collapsed back to medieval times" as a good ending.

In particular, there's nothing about our "long term potential" that requires us to get off Earth today, or even in the next century or two - that's why it's called long-term potential.

This is the same thing as extinction. If humans don’t escape earth, then with probability 1 we will go extinct relatively soon (relative to the lifetime of the universe). If not from a standard extinction event like meteors and volcanoes, then from the sun going supernova (ETA 5 billion years)