[dfxm12]

The problem is that answering this depends not on some objective notion of simplicity, but on a realistic assessment of /where time will go/ during the development process.

I don't think anyone mentions time as a proxy for simplicity. At least, the article certainly doesn't. You're right that the author doesn't objectively define simplicity, but I don't think anyone can. What is simple tends to be different to different people/teams, based on skills, tools, etc., available.

I know what's simple to me. I know it may not be simple to you. I know what's simple for a team in my org and I know it may not be simple for another team in another org. But, I do know what skills someone in my position and in my org is expected to have, and I know what tools are available to us, so I can make some real assertions here about what is "simple". Get worried beyond that, and you get bogged down on unknown unknowns.