I haven't read the link properly, but its notoriously difficult to apply statistics to football. That said, interesting that 'Figure 4: Best performing players at the end of contract' unsurprisingly features mainly older players, but the only one under 26, André Ayew, has just signed for Swansea. Swansea being one of the EPL clubs that do actually seem to have a sustainable recruitment strategy.
The next youngest, Alberto Bueno, has just signed for Porto.