Airbnb works really well for the awkward space between short-term and long-term. For example, if you're spending three months in Bangkok -- you'll want to have the features of an apartment (full kitchen, laundry, etc.).
The other niche is rooms for rent. This is primary used by young and cash-constrained folks -- but it's a surprisingly large number of bookings on Airbnb.
Personally, I think having more travel options for young people and the less wealthy is a hugely positive development.
Not much different from wanting to eliminate poverty from the world, however, we know full well a middle class person had greater impact on the environment. They consume more goods and waste more.
It's neither good or bad but something we must keep in mind as we can't wish the consequences away.
It's similar for automobiles. Take a look the overall carbon footprint for automobiles. Last time I looked it up, at least 1/3 of the car's lifetime carbon footprint was due to initial manufacturing -- for an asset that (as a personal automobile) has sub-1% utilization. Share the car, and we have fewer cars and tolerate >30% more individual travel for the same net carbon footprint.
Also, at the destination, more people puts pressure on the ecology there. Imagine everyone being able to afford going to Antarctica, or let's say, the Grand Canyon or Mt Fuji. It's be environmentally disastrous. We'd eventually have to regulate access as is currently done for places like Antelope canyon.