A better term is "non-essential complexity", since "accidental" implies that it wasn't intended. The reason it's a durable trend for non-essential complexity to increase is that there are common and powerful forces that push for increased non-essential complexity.
The prime example is short-term technical debt taken on to speed the release of something new. It's not accidental, the team intentionally chose to (temporarily, hopefully) take on the added complexity.
It doesn't generally help to think of one word as another word. That doesn't tend to work out well when trying to parse semantics.
"Incidental" is correct here, but not ideal for communication because of the eggcorn with "accidental". The word "accidental" is wrong no matter how you slice it.
The prime example is short-term technical debt taken on to speed the release of something new. It's not accidental, the team intentionally chose to (temporarily, hopefully) take on the added complexity.