Hi guys,
The phrase ever less likely suggests not just more unlikeliness but rather increasing unlikeliness.
Thus, I think the example would better read If such a tremendous number of young people continue to be lost each year due to the disease, our nation will become ever less likely to develop economically.
A couple of examples from the 'Net that show this increasing aspect (with my italics) -
However, as you wander farther off-topic . . . you are less likely to get a useful ansswer and ever more likely to generate resentment among the people who want to help. . .
The more the government interdicts, the harder and more expensive it becomes for refugees fleeing persecution to break through the government's defences to find protection in Canada. Women, especially women with children, are ever less likely to be able to make it through
To suggest 'less likely than before', but without this increasing aspect, one might use the phrase 'even less likely'.
Best wishes, Clive