It's crucial for a team to be able to judge its own performance and to look for ways to keep improving. And if solutions are identified, like training, travel, team building, it needs to be given a chance at it.
However in case the team still isn't working as well as it should over time, it's essential to have the courage to ask the lower performers to move on.
It's difficult but there are ways of making it a dialogue rather than a unilateral decision. You could coach the person to find another job or organise team self assessments.