Its not that humanity will go extinct with 1.5 degree rise in average temperature. However this will likely directly kill 10s/100s of millions of people and destroy ecosystems and weather patterns that humans depend on, which in turn can create a lot of political instability which could increase the chances of a extinction threatening event (like nuclear war for example). The other path to extinction is the world where we open up some feedback loops which are currently unaware of , runaway heating is an example of that, where each degree of heating causes the rate of heating to rise.