(We could develop the technology without going to Mars, but it might be harder to fund tech for "what if Earth's atmosphere becomes unbreathable" to completion long before that's an issue.)
If we stopped using all fossil fuels TODAY (as well as eliminated the cattle and hog industries), as in every single person on the planet, it would a few thousand years to return to pre-industrial revolution levels of CO2. We're realistically looking at a MINIMUM of 6ft increase in sea level before the end of the century (bye-bye most coastal cities unless you ring every last inch of coast with dikes). We are rapidly depleting aquifers the world around (much faster than they can replenish), we're rapidly depleting soil (creating appreciable amounts of topsoil takes centuries), population is still climbing, electricity demand the world around is climbing, someone is producing a banned ozone depleting compound ( https://arstechnica.com/science/2018/05/it-seems-someone-is-... ) etc.
It's not a lame reason, it is an extremely valid one. We need to develop the technology and means to have self-sustaining colonies (even if enclosed in habitats) off of Earth before stuff gets so bad here we simply don't have the resources to commit to doing such.
We're a long way from being Venus
I'd love to see humans achieve interplanetary colonization in my lifespan, but that one is a lame reason: as much as we hurt Earth, it will remain much more habitable than Mars.