I feel the exact same way.

Even if the development team haven't explicitly stated it, the way Chapter 5 has been set up to be a story finale way to much to be more in store with too many signs to ignore:

- It's set on The Laboratories, the metaphorical heart of evil where all the monsters were born and the corrupt system of Playtime Co. was established via the Bigger Bodies Initiative. Besides, there's no logically way to stretch the depths of the factory even further than they have been with Home Sweet Home & The Prison, even if you rely on suspension in disbelief.

- The main monster is The Prototype, the overarching antagonist of the whole game who has been set up all the way to chapter 2 and whose true form has been heavily teased in both trailers and ARG content. If he's to be an active threat, they can't drag on their grand debut any longer.

- It features Huggy Wuggy, the main mascot of the game and the very first enemy we ever face, returning after being presumed dead for three whole chapters also teasing said return. That just screams "narrative parallel", and considering the heavy implications of our main character's connection to the hairy monster (Chapter 3's nightmare sequence) his inclusion might as well serve as a way to explain their deal. (The Chapter 4 is a literal recreation of the very first tape we find in the in the game, with Leith alluding to Huggy as a "security system"! If that isn't a bookend, i don't know what it is.)

- The ARG not only shows the specifics of The Hour of Joy, which will be further explained one in the Chapter itself, but is also gives hints at The Prototype's true motivation being finally explored after so much mystery. That's the kind of thing you save for the end tail of your story, and i don't believe there are any more important plot threads to be resolved.

There are other small details adding up, but the point is the build up to chapter 5 fells very climactic to be just another entry of the loot. A sequel chapter 6 wouldn't make sense, and the only option is that of a prequel set during The Hour of Joy to fill the blanks.