'Ferrer can beat all of them'
Roger would disagree (and Nadal as well when he's back)
Well, technically, Ferrer has already beaten both Djokovic and Nadal. So he definitely CAN.
Regarding Federer, though, it's really speculation, because the 13-0 H2H suggests that he cannot
Not that DelPo's record against Roger is much better, 13-2 is pretty lopsided, but he has proved he could. He hasn't beaten him since 2009, though.
Now I know you're going to say "when he's 100% fit" etc... But that's speculation. Being fit and staying fit is also part of the tennis player's "job".
For all I know, Coria could have been one of the greatest clay court players, maybe he could even have given Nadal some trouble, if he had not had problems with cramps or with his shoulder... But his physical condition was his weakness. So I could still claim he was potentially as good as Nadal, there will never be a way to know if it's true or not.
Not that I want to take away from Del Potro, he remains the only guy who has taken a slam outside the Big 3, that's a fact and it's definitely a big achievement. But he also hasn't been further than a QF in a Slam since then.
So, maybe we're going to witness his great come-back, that's all we can wish him, maybe it will happen in this USO, but until this happens, I don't see why people always imply that he's so superior to other players who are better ranked than him.
Yes, he can beat top 3 players, just like Ferrer or Tsonga can. But for the moment it remains pretty unlikely that he can beat several of them in a row again.