(Notice the blue sunset at the 22 second mark? We're not in Kansas.)
A quick point: eco-people have been predicting dire events -- global catastrophe, apocalypse, etc. -- since at least Malthus (an early Greenie?), and have been proven wrong time after time after time. Hasn't stopped them from continuing to generate their "scary scenarios", and of course there's always a market for that sort of "End is Nigh!" sandwich-board advertising anyway. But it has at least helped many people see their proselytizing for what it is.
They do, though, have one ultimate point that rests on logic, not events: you cannot grow indefinitely given a finite resource. Mars -- and space generally -- is the logical answer to that logical point.
(The emphasis in the post's title, by the way, is just a result of a bumper sticker I remember seeing a while back, that had some allusion to the "limits to growth" and the line "Mars is not the answer". )
Tip to Speculist