Too big to fail usually indicates that the government will step in to save a bank/company, if there are signs that it could fail.
That definitely is not the case with Bitcoin.
Exactly, quite the contrary, I'm sure many governments and companies would like to see Bitcoin fail so this doesn't apply here. Either way, the main concept is understandable, there's too much money involved for Bitcoin to fail, but that means right now, this won't be forever. So Bitcoin must start to generate some actual demand, in today's world no common person actually needs Bitcoin (they can buy anything instantly with CCs and Paypal) and that's the real problem it could face in the near future.