system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why planets and moons have been on their orbits for millions of years, but James Webb Telescope needs fuel for only 10-20 years of orbiting L2?
gpt
The orbits of planets and moons *do* change over millions of years, but they'll still be orbiting around their respective primaries (stars and planets). For instance, the size of the Moon's orbit around the Earth is expanding about an inch per year.

The L2 position is unstable. Webb is orbiting around the Sun, and it will continue to do *that* indefinitely, but it'll wander away from L2 without repeated corrections.