there is a plan 'b' - its to not grant further ELA (physical money supplied by the rest of the EZ) coming out of Greek cashpoint machines.
the Greek banks are within two or three days of running out of physical cash, capital controls mean that direct bank transfers are banned, and Greek retailers and businesses stopped accepting credit and debit cards months ago due to the high charges and concern that if you don't have the cash in your hand, you don't really know if the money will be available for you to spend.
within a week or so there will be a significant sortage of food in the shops, and people won't have the money to by it anyway. medicines are already in short supply and petrol will become unavailable and unaffordable. the Greek government, even if it makes no payments to any creditor of any kind, has about two weeks of money left for pensions and wages. it has no money to buy food, medicines, fuel etc..
it will get about a weeks grace from the public while talking about this being an eeeeevviiiiiiiiiillllll EU plot etc.. but when the food runs out (Greece grows about 40% of its food needs) the Greek people (understandably) won't care about who's fault it is, they'll just want it fixed.
the Greek government will then face a choice - get money from somewhere else, or cave to the EZ with far worse conditions than those rejected by the government and referendum. they'll choose to go elsewhere and that means a return to the Drachma, EuroDrachma, RoubleDrachma or whatever. its worth will be around 1/3rd of the value of the Euro, and no one outside Greece will touch it with a 50ft bargepole.
plan B is not to kick Greece out of the Euro - the structures for that don't exist, and it would require unanimmity among the other Euro nations that isn't there (the French for example, though for reasons of self preservation rather than a principle of solidarity with Greece) - the plan B is to allow Greece to fall to a point where its government decides that it cannot remain.
this, for Germany, is not about Greece. its about the two, three, or four countries next in line who would want the same conditions that Syriza wanted. Germany could afford to keep Greece afloat, it could not however to keep Greece, Spain, Italy and France afloat.