I don't mind the smile staying... but I wonder how is the author going to use it.
Is the slime going to change sides or stay an asshole?
At least it will get to witness the Hero's speedrunning from now on so it will never get bored.
Each manga volume adheres to a page limit set by the publisher, and it will never go above or below that. That's why some manga will have paddings in shape of character illustrations, glossary, or a simple title drop... while others will be crunched so hard that there's only 6 extra pages added to manga volume compared to the magazine raws.
Note that this is just my personal observation, so it might not be what's actually happening in the field.