If you start greenfield and ignore the code quality, how do you know you can maintain it long term?

Greenfield is fundamentally easier than maintaining existing software. Once software exists, users expect it to behave a certain way and they expect their data to remain usable in new versions.

The existing software now imposes all sorts of contraints that may not be explicit in the spec. Some of these constraints end up making some changes very hard. Bad assumptions in data modeling can make migrations a nightmare.

You can't just write entirely new software every time the requirements change.