"Even if we are already locked in for significant warming over the coming century, we still need to do everything we can to reduce our emissions. Because 2 degrees is much better than 3 degrees, and 3 degrees is much better than 4 degrees."

That's not necessarily true.  The truth is you have no idea what the world will look like in 200 years or if that will even be a problem to humans living at that time.  Think about how much the world has changed since 1817.  The world is changing even more rapidly today than it was then, and the Industrial Revolution was the single greatest society-transforming event in human history.  

By 2217 we really don't have a clue what kind of scientific advancements or technologies they will have to mitigate or solve the climate change issue.  It may not even be an issue.  They could be geoengineering by then.