Um, GPL is not “closed source.”
If you're working on something that you hope to (eventually?) make money from, but you also want to benefit from the collegial atmosphere of open source, then choose the GPL. This simple decision instantly eliminates a whole subset of would-be competitors, who you now don't have to worry about undermining your business or otherwise threatening your source of income while they rely on being able to use your own work against you, since the GPL automatically places your work in their do-not-touch category.
This is true, and yet your conversation partner still has the winning answer. The point is that, notwithstanding these restrictions on developers—some of whom have the desire to be able to take without reciprocation—the freedom guaranteed by the GPL to users more than makes up for it. We're essentially talking about local versus global maxima here.
> It's almost impossible for a commercial product to use GPL 3 licensed software at all.
On the contrary—GPL is very beneficial, for the reasons I just laid out in another comment[1]. GPL-licensed code is not ipso facto any harder to commercialize than if you were to make the end result available under MIT. It's arguably even easier, since your competitors will have a more difficult time competing with you than if you'd chosen MIT. As Eben Moglen once put it, if you're trying to decide on a license, then by all means you should go with a permissive one—if you're what you're looking for is "a really good license for your competitor to use"[2].
You need only look to history then compare and contrast it with the industry's current preoccupation with funding the development of free and open source software. Red Hat/Cygnus were largely built on top of GPL code. Meanwhile, today's developers entrenched in the GitHub culture, with its permissive-first obsession with licenses like the MIT, have been bamboozled into working against their own interests by giving away their strongest bargaining chip when they decide to go with the flow instead of choosing the GPL.
And super permissive licenses like MIT hurt developers too, because it means I oftentimes can't see the source code that someone else has modified, even though it would help me a lot to be able to do so. With GPL-licensed software, I am always guaranteed this freedom.