Don't underestimate the power of human curiosity.
Once we do discover an exoplanet rich in oxygen and methane, I feel pretty sure it will pique the curiosity of some organization or country enough that they will devote the budget to make a massive array of space telescopes. Once we focus our attention on that area, we might also be able to spot other signs of life, by developing new techniques.
Call me a techno optimist, but it seems inevitable to me.
And it will change many things on this planet, when we do make that discovery.