Last time a new democratic Left party came to power in Greece, it was PASOK with Andreas Papandreou in 1981. Papandreou promised more radical things than Tsipras does. He promised to get Greece out of European Economic Community and NATO. He did neither of those. Instead, he had minor concessions from Europe and declared that the problem was solved. After that, he embarked on building a new clientele for his party by means of populist economic measures. He presided over several economic crises, but the Greek economy was generally in better shape than today, with low debt and its own perpetually depreciating currency. And yes, Social Democrat as he was, Papandreou was not shy of strict austerity when it was necessary (for example, during 1987's crisis).
Now Tsipras has some leverage to press Europe on new loans, because a Greek default would still be catastrophic, but ultimately Greece will come to heel, because it is such a small country that is afraid to fall out with Europe. Centrists have done much of the dirty job, so Europe will be able to give Tsipras some minor concessions to help him save face. Left cannot borrow, nor could they print money, and an exit from the Eurozone is technically impossible. That means they will be busy mostly with distributing those money that still are there to distribute. More lean clientele than PASOK’s, less sharp austerity, but otherwise pretty much the same thing – no economic growth and no visible perspective.