Why?

Because one of the main payoffs from getting here to the nearest habitable star system is that you're now closer to more habitable star systems.

But if turns out that it's very very expensive to get from one star system to another, then that recursive payoff might be very close to zero.

To avoid the issue with the sun frying the Earth you'd only need to move a little out in our own system. Mars or Io or some such.

Musk is talking about a "cargo route" to Mars starting 2018 so 100 million years is quite a while to do things. http://qz.com/705299/elon-musk-is-building-a-supply-chain-to...

Are you saying you think we have to completely deplete all resources in a stellar system to colonize the next? There is way more energy to harvest in a star system than that.

It can take more energy than it's worth to transport energy from one star to another. Entropy is a pain, because it's like we're trying to find a relatively dry tree to hide under in a rainstorm and none of them stay that way for long.

If the cost of transporting mass and energy from one star system to another are sufficiently high, another problem is that only the people on the ship get a benefit, whereas everyone else is throwing away mass and energy for no conceivable personal benefit.

It would take a very dictatorial sort of government to overcome this kind of negative incentive.