While you're probably right, I must point out that two years ago no one in Greece could have publicly predicted that SYRIZA would evolve into SYRIZA without suspicion that they are on crack. SYRIZA was polling around 3%, 15 months ago...

The IU -> SYRIZA scenario would go like this: First PSOE would have to start splintering. Then a left/sane faction would have to join the IU. Then the IU would have to seek alliance with other credible forces that would prop up its street cred which could be built by unwavering, clear and principled support of popular movements against austerity. That would involve removing any people who are too soft on the PSOE and recognizing that whatever remains of the PSOE is a political opponent, pretty much on the same level that the right is.
Then a leader with some public charisma would help, but is not really, absolutely necessary...
The problem is that at this stage there is no time for this scenario. Developments will occur at too rapid a pace so who knows... How strong is the fascist right in Spain right now and how high the level of xenophobia?

The road of excess leads to the palace of wisdom - William Blake