On choosing eternity and the 'return' in adventure genre:
If you've read quite a bit of children's and young adult literature, or been familiar with the adventure novel, and especially if you've studied these genres, there's a structure to it that you may be familiar with.
Whether it's something like your Robinson Crusoes or all the way to stuff like Wizard of Oz which may seem closer in comparison to Deltarune's genre (more in the Wonderland territory) these works have never been as simple as what they seem on the surface, of a heroic character going through hardships in a different world and being victorious/learning a lesson. They have elements throughout of a rhetorical nation and identity-building that makes one aspect of the structure very important: The Return.
The protagonists will often start in a familiar place, importantly something homely, something that is for the Family, and something that represents the nation on a microcosmic level. From there they are thrown into a different land: it is exotic and it is challenging but ultimately it is there just to build character for these protagonists. Ultimately they have to leave it behind and remember, as Dorothy does, that "there is no place like home". And these are elements less subtle in a book like Robinson Crusoe for eg where the "other" world is more clearly representing all of the "uncivilized" world against which the image of the Home, of the Nation, of the civilised world is created. Similarly with Wizard of Oz starting at the Homestead - a symbol of American domestic life that Dorothy very clearly shows to be "boring" but ultimately is the place she must return to. That is ultimately always what the adventurer, the protagonist is working towards: to return.
The thing that drives the heroes forward within the dark worlds in Deltarune each time is the need to close the Dark Fountains: to return home. Home is the base around which the adventure must be: the home is Real. The home is what you really are. The home is civilised world.
The land of the adventure? Yeah that's where most of the story is happening, but it's not real. It's the Other, the Exotic, the Uncivilised, the world that you enter and get to play around in and decide the fate of and then you return, leaving all those high stakes behind. The world that is real is defined against the unreal.
And traditionally, as I'm sure you see here, this structure to the adventure novel has had quite oriental and colonialist implications, there is a horde of literature that talks about this, about how this 'other' world often taking elements from other cultures is denigrated to a two-dimensional level of just serving the adventurer's purpose (like the Darkners serve the Lightner's purpose) before being put down against something better and more Real - the Home, and in this case literally named Hometown, the Light World.
Deltarune takes the adventure novel trope and really names and shames it for what it is: the othering and exoticising of a world that's meant to serve you and have no interiority of its own beyond that and not be seen as real. And though Ralsei, knowing the "rules" of this world, bows down to the trope (like Friday in Robinson Crusoe the noble savage, like the Scarecrow and the Lion and the Tin Man in Wizard of Oz furthering Dorothy's purposes) Susie does not see it as common-sensical.
Susie cannot take it on face value as just "how things are". So when Susie says she doesn't want the story to end, it's more than just holding on the fantasy.
She is rejecting the adventure novel structure.
She is rejecting the return.
She is saying fuck the expectation to let this other world and these other people burn just because they're not "real" enough, fuck this subservience and closing it up to back home.
Others have pointed out that Toby is unlikely to just repeat some old tired "too much escapism is bad" trope, that isn't where the story here seems to be going. And I think it's particularly about placing the story in a tradition of this kind of trope and structure and then ripping that expectation out from it and challenging the vantage point from which you view worlds that you just enter to "have adventures" in.
Just like Undertale challenged the idea of a world you enter where you view all residents without interiority and use and kill them for your gains because they're nothing more than NPCs in a game.
Deltarune challenges the idea of creating an "other" that it is acceptable to not give interiority to, that it is acceptable to treat as less than, that it is acceptable to leave behind after you get all the development for yourself you could get out of it. When Susie chooses eternity she rejects that quiet acceptance of the other as something to be cast aside.
And if nothing else I hope this will be the kind of rejection that can stick with people when they engage with the adventure genre and see its historical issues with this quiet colonial attitude that carries on to this day.
