Here's a thought.
If a Portuguese child had gone missing in the UK on holiday, would :
1. The case still be in the news ?
2. Would posters still be up after nearly 10 years ?
3. Have posters on billboards never , ever been vandalized before ?
4. It isn't Madeleine, the people who did the vandalizing are angry with, it's her PARENTS.
Defacing posters publicising a missing child really is the lowest of the low. The defaced image of Madeleine John posted was from 2009.
In the hypothetical scenario you suggest, I doubt very much if it would have been necessary for the parents of the missing child to employ private detectives to pursue their own investigation into their missing child's case.
The police would have attended to each aspect of the child's disappearance in exactly the same manner in which all cases of missing children are dealt with, obviating the requirement for the victim's family to devise their own strategy.
On those occasions when police investigations are unsuccessful absolutely no-one faults the families for keeping the case alive. In Keith Bennett's case all his mother wanted was for his body to be found so that she could bury him properly ... no-one castigated her for that, nor would they.
Why are the McCanns castigated for trying to find their daughter who may well be alive ... why are there campaigns to stop them from looking?
Weather-beaten memorial to Moors-murder victim Keith Bennett at Wessenden Head, West Yorkshire