"In spite of his calculations being plausible, I think Hawking has made another false assumption: that it is possible to travel to other extra-solar planets. It isn't."

If humans wanted to travel to extra-solar planets, then they could do it. It would take a lot of work- not millions of man hours, but trillions of man hours.
Travelling to a extra-solar planet would similar to building a bridge [or tunnel] across the Pacific ocean. You could say this is impossible because there is not enough people that want this done- and there are very good reasons why there is a severe shortage of people who desire this. But if enough people desired this [if there was the political will] the next question is how would this be done {of course, doesn't have to follow this sequence: how it could be done could lead to a political will to do it- and that is generally the more common way something actually happens}.

That it is possible to travel to extra-solar planets is not the same as settling an extra-solar planet- we have travelled to the Moon, but no one is living there at the present time. It is unlikely that we will colonize an extra-solar planet before colonizing such places as the Moon, Mars, Mercury, and/or elsewhere in our solar system.

Establishing any kind significant of presence elsewhere in our solar system will require billions of man hours. A rough metric of today is that 10 million dollar would a be cheap ticket to orbit, or $200,000 is cheap ticket to suborbital space, or 200 million dollars would cheap ticket to the Moon or Mars. At such time as a suborbital trip is $10,000 and orbital is a million dollars and Moon or Mars is 10 million dollar, I would say we would be 1/2 way to a significant presense in space. $1000 for suborbital, $100,000 for orbit, and less than 1 million dollar for ticket to Mars or Moon or elsewhere would indicate to me that there was some kind of significant presence in space. At this kind of level of space development, going to a extra-solar planet would be roughly equal to going to the Moon in 1960 and colonizing a extra-solar planet [implying some kind lasting relationship] would be like colonizing Mars in 1960.