Fossil fuels aren't necessary. When we harness renewable energy sources like solar power and wind we won't need them. If we ever get a mastery of fusion we'll have more energy than we will ever need. As for metal ores, everything can be recycled, and it's not like the metal is going anywhere. Plus, I could see us harvesting the resources of other planets in the future, although I believe that Mars is devoid of natural resources so it would most likely be asteroids that would be mined. If we're getting even more hopeful for the future, we could just make artificial metals. All metals are made of the same particles, just in different amounts. The only finite resource that we have is space, but we have a lot so it would take a lot of people to take that all up.
We're still a very long ways off from being able to efficiently meet most of our energy demands on solar/photovoltaics and wind power alone. Solar and photovoltaics require areas that get plentiful amounts of sunlight throughout the year, so if you live up in Washington or in the northeast near Canada, you're not going to get much out of them. There's also the problem of storing it whenever there's no sunlight available since it's not a continuous source of energy. Furthermore, its high cost keeps developing nations from being able to afford it, and as a result they have to rely on conventional fossil fuels. Plus, countries such as China and India are still heavily dependent upon oil and coal (China especially, as they're the biggest consumer of coal). So until efficiency is increased and cost is reduced, it's not going to make much of a dent in fossil fuel consumption for awhile.
Wind power is less expensive, but it faces a similar problem in that it's an intermittent source of energy, so that's an issue when power output is low and energy demand is high. It also comes with some political baggage because it causes noise pollution and is seen as aesthetically offensive to people who live around them, and it's pretty common for people to lobby against their development. Although, offshore wind farms may be able to circumvent this.
As for fusion, I'm not holding my breath. We've been trying to get that to work for decades. Back in the 70's it was predicted that we'd have a few commercial plants up and running by now (my dad almost went into fusion research, in fact). That area of research hasn't seen much progress for a very long time.
Although, liquid fluoride thorium reactors may prove to be a viable alternative to conventional uranium reactors. They're vastly more efficient since they consume almost all of the thorium fuel needed for the reaction, and the thorium itself is far more abundant than uranium. This was something that my waves and optics professor mentioned last semester, as his research team found a way to make mostly pure lithium-7 by removing the lithium-6 isotopes that would interfere with the reactor start-up process (you need ridiculously pure samples of lithium-7 to sustain the reaction). So a couple of nuclear energy companies got interested in his work and contacted him about it, and he told us about how we might end up with smaller scale reactors powering individual cities and towns. I haven't done much research outside of what he told us though, so I don't know about what kind of disadvantages it has.
With all of that said, the transition to sustainable energy is still going to be a long and arduous path since we're heavily dependent upon non-renewable energy to maintain our energy grid and methods of transportation, and this is going to have serious economic consequences since we've pretty much dug ourselves into a hole. But again, the biggest problem is developing nations that are attempting to meet our standards of living that won't be able to afford the development and implementation of more renewable sources of energy.
Mining planets or asteroids is so far off and has so many logistical obstacles to overcome before we can even consider it a practical possibility for the foreseeable future. So for now, I think we'd be better off focusing on what we've got to work with down here on Earth.