Hello Malathi Ajay,
Autumn Jane is absolutely right--the probability of any set of employees becoming mediocre can be for a variety of reasons.
Even though the PRESENT team MAY BE exemplary, human nature being what it is, the tendency to 'take things for granted' OR 'feeling indispensable' OR 'plain complacency' CREEPS into most people over a period of time. It's better & wise to preempt such situations arising rather than giving scope & then go about handling them @ a later point of time.
Another reason--especially if the core strength of your company is in Technical areas--meaning the company's business depends on upgrading/updating/enhancing technology on a very regular basis [IT, Automotive, Telecom, High-technology manufacturing & some other Sectors fall into this category], then attrition is good in the long-run. The interest AND convenience for any human being to upgrade/update/enhance his/her technical knowledge-base regularly without interruption usually reduces with age--NOT out-of-choice, but due to changing priorities in life. This then leads to having, say after 5/10/15 yrs, what Autumn Jane mentioned: "a pool of mediocre employees".
However, this DEFINITELY DOESN'T MEAN that attrition should be ignored, since this aspect of HR has the associated costs & effects on the organizational well-being. The trick would be to know where to draw the Line between What's Acceptable & What's Not.
This, obviously, gets reflected on & in the scope & profundity of the Training policies of the company--but that's another story.
While involving the Team Leaders in this exercise is good--draws them into the HR process, I have a question. Has any effort been made to identify the Teams where this problem is noticed/experienced MORE/LESS? If it's across-the-board, then the problem COULD lie elsewhere--and the chances of the Team Leads feeling disheartened later is high. Suggest look into this aspect too.
All the Best.
Rgds,
TS