A well-built software system doesn't have to grow in complexity over time. More features does not equal more complex in the true sense of Simple.
Faulty data models and system designs -- that aren't fixed -- lead to ever-increasing complexity. But that is the fault of the data model/designer.
I.e., there is a way to build (and grow) systems w/o linear increase in complexity -- but it takes a particular rare skill set.
And by that I mean, it has to account for more scenarios or do additional things... unless your software is growing in complexity while you're only removing features...
Bugs in software come from thinking we're simplifying the world in one way though the program, while in reality it receives a slightly different picture.