Greece's debt was restructured to be drawn out into longer payment terms at a lower interest rate and at a reduced principal to be fair to them and to prevent a financial collapse.
It's containment. It's been 5 years of containment now, and it's working to some extent as nations within the eurozone are much less exposed to the greek situation than they were 5 years ago.
Check this prescient article from 18 years ago by one of the best macro minds of recent years (Rudi Dornbusch):
https://www.foreignaffairs.com/articles/europe/1996-09-01/eu...
He talks about excluding Italy, Portugal, Greece, Spain, etc. So what happened? Standards got relaxed and they accepted e.g. Spain. And then other countries completely falsified their numbers and also got in due to that (Greece).
I don't even know why are they still thinking about keeping Greece in the Euro. They don't even have the internal political capital to make the deep reforms they need (labor mkt reforms, an IRS with bite, retirement at a much older age, etc). It would have been better for everyone on the first place to assume Greece will devalue, and to start planning for that.
They default, have maybe half a year of real pain, and then they get back on their feet albeit with financial autarky.
Like they did with the EMS. https://en.wikipedia.org/wiki/European_Monetary_System
It didn't work at the time, they now wanted to make it forcibly work (again). Something is not right.
As for your question. If Greece leaves the Euro then it is the end of the euro zone. When Florida was going to default the federal reserve paid for them. That way people has confidence in the dollar. If when a European country is going to default every body screams and runs people knows that they shouldn't have any confidence.
No. Greece is too small for its default to be that important, especially given that the vast majority of debt is actually now in the hands of governments and government institutions.