What‽‽‽ There are so, so many reasons for space migration that have nothing to do with earth's environmental problems – assuming you mean global warming/dimming/<things humans are affecting>. Asteroid. Super volcano. Gamma ray burst. Magnetic field reversal. Pandemic. Super fungus. Artificial super intelligence. Nuclear war.
Our understanding of life, geology, climatology, astronomy, etc., has improved much in the last 6000 years, but at a fundamental level, we are extremely rudimentary. We don't really know what's normal on a galactic/universal scale.
It's like saying having a failover server is just an expensive way to sideline taking good care of your primary server. Huh? No, having a backup plan is what you do when something is important, because shit happens. On a long enough timeline, shit that you can't predict always happens.