There's almost nothing we could do to this planet which would make Mars a more attractive option. I guess there's always a meteor/comet risk, but practically anything else still leaves Earth as a better option.
Even if we all had to live in bio-domes because the planet was so polluted, it would _still_ be a better option than Mars, just by virtue of the fact that we have gravity and an atmosphere. On Mars, a cracked bio-dome means everybody dies. On Earth, you could probably just go fix it.
Earth has:
- Gravity
- Much better temperatures compared to Mars
- A moderately-pressurized atmosphere
- Water (even if it's salty/polluted)