Is there a way to have EVERYBODY on the watchlist by default?

Another option is to set Escalations for your SLA(s) as opposed to using a Trigger (that's what I do). Of course this is specific to each Services service level agreement. Example, all my SLA(s) for a Service are configured to Escalate when due date / time is missed. This sends an Escalation notification to the Service Admin (me) and the Assignee. There also is a good amount of escalation options foe each SLA.

If those out of box configuration options don't work we can then look at Triggers. Unless you really need to add a comment.