Everything is impossible until somebody goes and does it. While going to Mars and beyond is going to be very hard with current technology, there are other possibilities. Even in situ resource utilisation, e.g. making rocket fuel on the moon, would make it drastically easier to get around the solar system.
Then there is nuclear propulsion, which would hugely cut travel times. These are technologies that will likely be viable in the foreseeable future. Then we have fusion, which will probably take a little longer. And then there after things that nobody can even imagine today. Like nobody could imagine a smartphone thirty years ago.