I am also waiting to see players like Sanju Samson performing in international arena but his fate is to get picked in the team and keep on warming the bench series after series even when major players are rested. When India chased a total of 200 there was Rohit Sharma in the team and can take on any bowlers and if Sri Lanka makes a huge target like that then it will be a difficult task.
I guess the senior players are afraid of losing their place in the team, in case some of these youngsters perform well. Perhaps that is the reason why they are not giving any chances for the younger players, even against weak teams such as Sri Lanka. Now one more match is remaining in this series, and ideally they should rest some of the seniors such as Kohli, Dhawan and Rahul and give chances to players like Sanju Samson and Manish Pandey.
I beg to differ from your analysis, i see things slightly differently.
Dhawan - He didn't play much cricket after the World cup due to injury and he's technically out from the Test cricket for good. So match practice is necessary for him.
KL Rahul - He's out from the Test Cricket and management trying hard to fit him at the top order in ODI/T-20. Rohit is taking rest so he is the first choice for opener and Sanju/Manish are middle order batsmen.
Kohli - His presence in the squad is debatable. one can argue that he should skip and give chance to younger (Sanju) player or completely opposite that he should play the T-20 match because Rohit is not plying/ Practice for T-20 world cup. i don't mind if he skips any T-20 series along with Rohit (if its possible). In their absense Shreyas Iyer can easily manage captaincy in T-20.
