We have all the energy we could use right now blazing away in the sky.. and thats just the tiny portion of the sun's emissions that are hitting the pinhead that is our planet.. its sending out that much energy in a full 360x360 degrees.
Leaving the toxic scraps of our energy solutions laying around the planet is not a long term solution, nor is it a particularly inspired one...
1. mine coal
2. burn coal
3. dump toxic waste on the ground behind a wall
4. ??
5. Profit!
It really is a question of when. Nothing lasts forever, and every civilization (barring the few current ones) has fallen. it may be 4000 years from now, but there may come a point where there isn't the organization necessary to maintain the jerry-rigged solutions we have developed.
Theres more than cost that makes throwing nuclear waste on a rocket to the sun a bad idea. It would only take one rubber valve to crack to spew all that waste out into the atmosphere. (space elevator would resolve that, or catapult)
I wouldn't mind nuclear if we could treat and truly dispose of the waste, not just hide it somewhere, but I think Fusion and solar are the future. Solar panels can be made with a computer printer, with the right motivation it wouldn't be hard for our societies to begin harassing sunlight where ever it shines. The siding on your house could have ingrained photo cells, your windows, your shingles. In 50-100 years i'm guessing that household energy will largely be generated on location with large power plants (possibly fusion) only used for industry and to cover any gaps. Power companies will be more brokers than producers. Your car will act as a back up battery for your house. People will be selling power to the grid, rather than buying from it.
One last thing before i stop rambling. One of the biggest things that will need to be done to advance on energy is to make advances available everywhere on the globe as soon as possible. If everyone who uses oil or coal now managed to move on to some other energy source, the same amount of oil and coal would be used, just by the people who couldn't afford it before. Its like when locomotives were first developed some guy in the business speculated that the price of coal would plummet as they developed more efficient engines. It actually turn out the other way, because as the engines became more efficient, people just ended up using the surplus coal in more ways.