If anyone else said that, I'd say "You're kidding, right?" It's a demonstrable fact that for similar projects at the same level of maturity, open source is superior to closed.

1) Open source is not "manhandled". Inferior modifications are simply not accepted by the maintainer(s). If someone comes up with better ideas that the current maintainers don't like, the project gets forked, the original branch dies while the superior branch thrives. Evolution in action.

2) Open source projects of significance gain from the knowledge and experience of a world wide pool of talent. Closed source projects are limited by the restricted abilities of a relatively small group of people.

3) Motivations matter. A lot. Work done by those who love what they do, and do it for their own perceived benefit, is almost always of better quality than those who do it solely for money. The two motivations are of course not mutually exclusive, but they do tilt the playing field.

4) Closed source projects are usually not directed by those who do the work. They're directed by people who almost always know virtually nothing about programming or engineering, specifically marketing and others with an agenda. Not that that's inherently bad--I'm a capitalist also. But it will result in products with more bugs and less quality. Too often products are shoved out the door to meet arbitrary deadlines, without any consideration as to whether or not the product is ready.

I could go on, but you get the point. Before responding with what you think are counterexamples, keep in mind that I said "at the same level of maturity". There is currently nothing remotely comparable to Mathematica in the open source world. But we're catching up, and the rate of catch up is accelerating. In regards to Windows, open source left them in the dust long ago. Microsoft is playing catch up but they'll continue to fall farther behind despite their efforts. Take a look at Vista.

Oh, wait, you said "will not have the same conceptual integrity". You didn't say which way it went. Never mind :-)