If Madeleine is dead, then she was presumably murdered, therefore the police should go on looking for the culprit for as long as he/she/they could reasonably be assumed to be still alive. Many murderers have been convicted 20 years or more after the event.
Furthermore, you cannot expect any parent to give up hope of their child being still alive, until a body has been found. Especially since there have been numerous cases of children and even young adults being kidnapped and kept prisoner for considerably longer than the time for which Madeleine has been missing. The three women recently released from captivity in America being the most recent notable example.
Whatever did happen, perhaps those giving money to the cause of the parents directly or indirectly, can give an equal amount to the Portugese police. This way, this thing will be fair and played on a level field.