Our dependency on them must be decreased, in order to sustain them for the future.
If we talk about reducing our dependence on fossil fuels, then this is the same as requiring enormous research and development. However, I highly doubt it can diminish in a short amount of time. It takes an extra approach and a very long time to change the people's habits that have taken root. If fossil materials are replaced, the solution is electricity, but electricity also requires coal for its basic material.
That's true, it is indeed a great idea to produce energy from renewable sources, however I don't think we have the technology to apply it on a large scale, yet. I believe that we'll have positive results the upcoming decades, increasing our renewable sources capacity will also decrease our dependency on fossil fuels, thus, preserving them for an extended period.