One thing that fuels my belief that Sasha & Marcy or atleast one of them will stay in Amphibia after the show. Is the fact that Amphibia is treated as a real place. In a typical isekai(or atleast the ones I've seen). The protagonist is spired away learns lessons then returns to the real world. There's stakes but the world is treated like a fantasy world. In Amphibia however they make sure to drill it into the audience that Amphibia is a real world with real stakes. So there was never really any escape form reality, just a change of location.
*But I'm just theorizing anything can happen at this point, and I'm very excited to see!!