I doubt it would be "for life", just until the fact that the data had been obtained would not prejudice the investigation.
I think the way it works is that the warrant states that you are placed under a duty not to disclose the existence of the warrant itself using a phrase such as "by any act or omission". A "warrant canary", as shown by its title alone, is a method of indicating that such a warrant has been served. Therefore by not publishing a valid "warrant canary" you would, by omission, have breached the warrant because it would make the existence of the warrant known.
IANAL, I am just trying to remember something I read a long time ago when the idea of "warrant canaries" first became popular.
Additionally, activate the "warrant canary" and BOOM the business is gone. It has declared itself compromised but it still cannot explain what or how much it had to divulge or why or to whom, without blatantly disobeying the warrant. It declares that potentially all users have had all their data seized even if it was for one file that customer x was known to have uploaded on a specific date at a specific time.
It's also quite a responsibility. You often hear of companies forgetting to renew their TLS certificates and the like. You cannot forget to renew a "warrant canary" and then say, "Ooops sorry!".
As I see it, the "warrant canary" is a self-destruct button for those whose business model requires them to have a self-destruct button. One which they are prepared to use at any cost both personally and as a company.
Remember a "warrant canary" is not a "fun" feature to have. This is serious stuff. We hear about warrants being abused with authorities being accused of too broad data grabbing or over-reach. However as far as those authorities are concerned a "warrant canary" is something used to unlawfully alert those under investigation, undermine an investigation and so potentially help them avoid being brought to justice. They will likely treat it as such.
Again, not a lawyer. Just someone who has been mulling this over since the topic was first created.