Actually, from my experience, it takes longer to rewrite code for an existing application, it's actually less work if they made a sequel, it also gives them more flexibility to keep updating the 2nd one while they work on the sequel.
In reality, not making a sequel is what's going to screw them over in the long term. Keeping the hype train going with Among Us 2 is a better idea.
They can just release a beta version of Among Us 2 in a few months to show their progress. It doesn't take a year and a half to make a pretty decent base game. (Of course, they definitely need to update it, but they could make a decent game without spending a year and a half.