As far as I'm aware it wasn't so much the ECB that were worried about the debt sustainability of the latest greek proposals as the IMF, who felt that more of the gap should be filled with cuts to benefits as opposed to increases in taxes.

The problem with not "caving in" to the demands of their creditors is that if Greece defaults, it's likely to have serious consequences for the affordability of their pension arangements. Whether those are more, or less, serious than the consequences of acceding to the requirements of their creditors is unknown.