Yeah Thirimanne has been a disappointment. Mathews and Chandimal will probably take over from Mahela and Jayawardene in terms of run scoring. And as you have seen both are highly capable. In fact Chandimal is Sri Lanka's answer to Virat Kohli. Talent wise Chandimal may be no comparison, but you ll see he will score plenty of runs (Like you cant compare Mahela, Sanga with the class of Sehwag, Dravid, Tendulkar, but both Mahela and Sanga have scored plenty of runs).
And the system is rather strange here. Players bloom very late. For example: Dilshan only peaked at 31. You get guys like Welagedara, Prasad, Samaraweera who have also blossomed very late. So there is every chance someone like Kapugedara who has flopped so much will pick up at some point. In BD the trend is opposite, players peak early and after 25 they pretty much become out of contention.
We ve always been better than Sri Lanka in age group competitions. But what happens in Under 19 tournaments doesnt really matter. Its how they convert these under 19 players to international teams. And not all their top players come from Under 19 system. SL coaches, selectors have uncanny ways of unearthing 25-26 year olds from out of the blue who have limited first class experience but have the skills for international cricket. With that a World Cup and 2 World Cup Finals in recent years. Not a bad record for a country which doesnt boast the talent of India and Pakistan.