I would normally agree with this and indeed is an argument I have used before. If we don’t react now however, then we are giving Assad and Russia a free ticket to use chemical weapons whenever they want. There has to be a price......
I said this in 2013:
https://www.grandoldteam.com/forum/threads/so-this-syria-thing.59719/page-18#post-2196475
Here's the thing though, I don't support a war against Syria. Nobody would. But there has to be a smacked arse for Assad as a consequence for using chemical weaponry. Cruise missiles, drone strikes etc. It actually doesn't matter if they're successful or not, it's solely the message it sends.
The term "red line" is a perfect metaphor. I couldn't care less if Assad went around gunning the rebels down, nothing to do with us, but in the longer term it's critical for the bigger nations to smack down any prospect of smaller nations going on a rampage with chemical weaponry and dirty nuclear bombs.
It remains true now.
Let's face it, it was Obama and his god-awful foreign policy decisions that has led to Syria/Russia being empowered to do what they are doing now. I really liked Obama, but he had weaknesses and foreign policy was clear as day a weakness of his.