Within certain restrictions. There is nothing wrong with players playing on if there is deemed a likelihood or probability of the game still being "winnable". If it is a matter of being held hostage by the players, then the circumstances of the game will have to be examined. (If one or two players have a considerable lead (or leads) and could be a 1 v 9 or 2 v 8 carries, then it is unlikely that a report would be taken seriously.) In a ranked game, it is hard to fault people for not surrendering - given that LP and MMR are on the line.
In the alternative, and this should be pointed out, forcing the end of a game is also reportable. For example, if you lose lane, and decide that the game should be over, leading you to force the end of the game by throwing (e.g., intentional feeding or constructive feeding) and repeated surrender votes - this is quite reportable.