The real problem isn’t Greece (or rather, the OP discussion problem); rather, the problem is how banks work. Banks work on trust - I cash a cheque drawn on Barclays at BankAmerica, they get the money from Barclays, I use a TDbank ATM to withdraw BankAmerica money, they get the money from them, etc. Direct deposit paycheques, mortgage payments, credit card bills paid and charges, etc. - all our transactions flow from one bank to another.
Each bank trusts that, based on government oversight, the other banks are not broke and will pay up when it is time to settle.
So Grece wants to pay its civil service paycheques and pensions. It goes to the banks involved. they say, “sorry, you have no money in your account”. If Greece were on its own currency, its central bank could probably just “create” more money. (Create too much, you get inflation…)
If one or more banks that deal with the Greek government decide to ignore this fact and hand out money anyway, eventually the rest of the banks will figure out these banks have no more money, they are bluffing. (Like writing cheques and shuffling money around to “cover” it.) If the other banks, not wanting to be stuck with the hot potato, stop trusting these lying banks, then the whole financial system collapses like a house of cards.
This is what almost happened to teh USA and teh world afew months before Obama’s inauguration. Every bank was worried that every other bank had so much bad mortgage debt, they refused to trust each other. All your banking stops working, since to pay a bill you have to get money, cash, and take it to whoever you pay your bill. They won’t take a cheque drawn on a different bank. They won’t give you a loan, since there’s no guarantee your employer will have the cash to cover it. and so on…
Normally, a bank in a temporary situation goes to the central bank and borrows enough to tide them over until the temporary cash shortage is over. For the USA, they simply guaranteed everything to get confidence going again.
In the case of Greece, if the European central bank is hostile, they are stuck. Eventually, the banks wills top honouring governmet cheques, payroll, pensions. Other banks will stop dealing with the Greek banks if they do not. If you can’t pay anyone for work or for goods, how do you operate? How do your civil servants eat?
Worse than that, if half your workforce is civil servants (plus pensioners) and they can’t pay mortgages or car loans - how long before the banks themselves go belly up? Taking all the local businesses with them, and everyone’s savings, etc.
Greece has to come to a deal with the central bank before that happens. For now, it’s almost like a game of chicken. WOuld the Euro bank let the Greek economy go down the drain, taking a lot of Europe with it?