The only evidence of abduction is that provided by Madeeine's parents. Therefore those who accept that story are relying entirely on the truthfulness of her parents.
You can apply that weak logic to almost any crime if there is no video of it and you want to assume everyone is just lying. In most burglaries for example, there is no evidence of a burglary other than the owner saying they have been burgled. But just because the only evidence of the burglary is because the owner "said so", does not mean that there is no evidence of a burglary.
Yes, they could have set it up themselves, the open door, the missing items, all as part of an elaborate insurance fraud because this is all circumstantial evidence. And if police have reason to suspect that as a motive, then it's reasonable to suspect they may be lying and investigate accordingly. But that is where the sceptics argument falters. What is the
motive for the McCanns AND the Tapas 7 to ALL lie?