We all know that The Travelers protected the Anchor right? But why? They killed Tessa for a reason and I doubt it was because she created immortality I think it was because she knew how to cure it. Same for wanting Silas to stay locked up he'd do anything to destroy the other side. The Travelers don't care about destroying immortality they want the otherside to stay up and I think I know why.
I think Two Thousand years ago spirits could come and go and possess who ever they wanted as they pleased then Tessa created the veil and that stopped it. However she was a liabitlity so they killed her.
It makes sense the other side is the only thing keeping Phantoms(evil spirits) from making the living world their playground. I mean an evil spirit is a witch's worst nightmare they can possess whoever may be even attack you with or without a host there is no defense against something like that.