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I think software quality is always decreasing because of multiple factors.

1) The tension between leadership that usually only prioritizes shipping new features and engineering that wants to write sustainable code. There is usually no incentive for engineering to decrease technical debt or increase sustainability, as these changes are usually not tangible to leadership. The only short-term beneficiaries of one engineer's code quality improvements are other engineers, which often don't care that much, as they are busy shipping new features as quickly as possible.

2) Speaking of short-term, not all leadership thinks long-term. Sustainable code makes sense if you're thinking long-term, but not if leadership is chasing lucrative exits in a couple of months. If a project survives for 20 years, chances are it survived multiple leadership changes, which all thought short-term.

3) In software, the only constant is change. As project gets older, it has to deal with all the changes that aging entails. Aging means increased exposure to reality and all the "surprising amount of detail"[1] it contains. This increases complexity, introduces bugs, adds edge cases to deal with, etc.

[1] http://johnsalvatier.org/blog/2017/reality-has-a-surprising-...