The warrant canary is not a solution to the problem of secret legal actions. It's merely a protest.
As we see the warrant canary idea institutionalized - with companies like Apple and Twitter adopting them to a certain degree, and cataloguing them on sites like canary watch[1] we are falling down a rabbit hole of speech about speech (and speech about speech about speech, etc.).
The real issue is NSLs and other secret warrants. These are older than you might think. Look up PRTT (pen register trap and trace, for telephones, which can be served with a gag order) or the All Writs Act. This is what we need to be working against, and this is what falls to the background as we find new and better ways to run our warrant canaries.
I am guilty of this.[2]