There certainly appears to have been sufficient evidence available both to the PJ and SY to enable the reopening of Madeleine McCann's case.
**snip
Det Ch Insp Andy Redwood, who is heading what has been called Operation Grange, said: "The review has given us new thinking, new theories, new evidence and new witnesses."
http://www.bbc.co.uk/news/uk-23179230
Without that hard evidence there is no doubt Madeleine McCann's case would have been reassigned to the dustbin of the archives where it had rested since 2008 and where it would have remained without her parents' much despised and actively resisted and obstructed campaign on her behalf.
The reopening of the case answers the question of the thread title.
According to the BBC story Operation Grange spent two years reviewing 300,500 documents. That would be 11,223 from the Portuguese investigation, 11,000 from Leicester Police and the remaining 8,277 from private detectives then? I would have thought that everything that Leicester Police had, the PJ also had? If not, why not? Certainly the Mccann's thought so, because they dropped their request to see all the evidence which Leicester Police had because the PJ Files were going to be released. Anyway, Op. Grange were two thirds of the way through these documents when the interview took place.
The review told DCI Redwood there was no clear definitive proof that Madeleine McCann was dead. Perhaps the proof was still hiding in the third he hadn't yet reviewed? Still, never mind that, the lack of proof of death means she could be alive! Great deduction there. DCI Redwood has also found 38 persons of interest, 12 of them British. We know that three of those re-interviewed were John and Donna Hill and Robert Murat. Do we know who the other nine were? Anyway, that would seem to be as much as Operation Grange were prepared to say about the basis on which the review became an investigation. If he had 'hard' evidence we have seen no sign of it in the two years since.
We learned more of Operation Grange's thinking during the Crimewatch programme. They had found 'Tannerman' and DCI Redwood is almost certain that he wasn't the abductor. They had two 'new' e-fits and although the programme hinted that they had been provided by the Smiths, it didn't actually say so. An 'Irish family' is mentioned who saw someone carrying a child. Can we describe e-fits as 'hard evidence'? I think not. The e-fits don't seem to look like any of the Portuguese people we saw going to be interviewed. Finally, DCI Redwood did an awful lot of digging for someone who was looking for a live child.
I don't know why the review became an investigation, but 'hard' evidence seems thin on the ground to me. Perhaps all will become clear, but I'm not holding my breath.