Exoplanet hunters' Graal:Earth's twin
What instruments such as HARPS (or an improved one) are likely to find, is the astronomer's Graal:
A planet with Earth's mass, one AU from its G2 type star...or Earth's twin. Hopefully, less than 100 light-years from here.
However, it's a spectrograph, not an optical telescope.
Once its existence is established, the scientific community will pressure politicians all over the world to build a telescope capable of imaging it and that would be the greatest discovery ever.
And all that could happen long before humans would step on Mars.