You're right to a certain extent.
I would have voted our own Syriza, if I had known before that Democrats would have had such a landslide.
Problem was that we're dealing with our own UKIP-like, Grillo's M5S, that at last year elections got 25%, becoming the most voted "stand alone" party in Italy (they're not the major party in parliament just because PD run in a coalition).
Mine, and I assume lots of other Tsipras votes, was a vote to thwart M5S success.
It should also be mentioned that during the last year PD underwent lots of renovations and almost the entire management was replaced by new and younger people.
While I mostly don't agree with Renzi (and at the primary for PD secretary I voted the far-left candidate), current PD could or could not be the "same shit different assholes".
(I re-read your post and I guess the first "you" meant a you as Italian people, not "you" me..
Oh well, my post should still be a coherent answer though.)