Though I do not claim the ability to decide between the rishonim that argued on this question, still I see the point of the Ramban and the Gra.
First I should mention that this was also more or less manifested in the Mir Yeshiva in NY. There it was the rule that the students would learn Torah all day and going to university was not an option.
I was in Safed for seven years and did not do much learning, but still I was doing some learning, and God provided. It was when I decided to go out and find work everything fell apart. Without going into the gory details, it ought to be clear that as long as I could manage to sit and learn Torah I ought to have done so.
The problem is that the Ramban states this idea of trust without effort in only one place --where he says this in reference to doctors. And there are plenty of routine procedures that are well known.
[This issue I do not hope to resolve, but I have heard from people that left the kollel system regrets about doing so. When I left it and consciously went about trying to find work people consistently complained about me that I was not working. The very same people who never put in an honest day's work in their lives. So if you simply look at the facts-the truth is cloudy. Lots of unworthy and insincere people take advantage of the kollel system. But does that take away all its positive aspects? I guess not. Where is any system that can't be abused?