The thinking at the time was that the planet was going to become irrevocably overpopulated and all resources depleted and so we would eventually need to create mining colonies on asteroids and other planets eventually.

Sagan was not keen on the exploitation of resources aspect but the scientific potential was too great to pass up. Also, being an astronomer, being able to physically and viscerally touch alien worlds would be an absolute dream to him, the human experience reigning supreme.