The problem is that it only takes one or two wildly incompetent people to completely disrupt the quality of the software. These are the kinds of developers who actively create bugs, usually by building (or copy/pasting) solutions that only work by accident, or who decrease the velocity of everyone around them by generating reams of overcomplicated and brittle code that is hard to test, hard to review and hard to maintain. It costs a lot of management time too, trying to find a way to get them to improve, or to build a solid case for letting them go.
I think the reason why every developer tends to have a story about these sorts of incompetent colleagues is not necessarily because 50% of their colleages are incompetent, but because even if just 2% (one person in the department) or 5% (one person in your larger project team) is incompetent, that can be enough to cause a seriously negative impact.
I think the reason why every developer tends to have a story about these sorts of incompetent colleagues is not necessarily because 50% of their colleages are incompetent, but because even if just 2% (one person in the department) or 5% (one person in your larger project team) is incompetent, that can be enough to cause a seriously negative impact.