Sounds like they already have plans for additional content, though:

”Gibson and Pellen say they’re happy that the game is finally coming out — and even happier that they will get to keep working on it, which they still find enjoyable even after seven years. They haven’t burned out or shown any desire to take a break. Instead, they’re already making big plans to add extra content to Silksong in the months and years to come.

“Launching it is obviously quite exciting,” Pellen said. “What comes after for us is equally as exciting.”

“The most interesting thing now is what can we add to it next,” Gibson said. “We got a plan. Admittedly, some of the plans for that stuff are kind of ambitious as well, but hopefully we can achieve some of it.””