Everything is "only guessing" to an extent. But some guesses are supported by argument. The guess that we'd be better off within the EU is because large alliances have more bargaining power on the world stage than a small nation on its own with very little manufacturing base and sky-high living costs.
The EU has handed over €38bn to help keep Greece solvent and if you include total loans, it's over €300bn. Do you think that would have been given Greece if it weren't a member of the EU? It would have been left to drown and sell off every asset it has to China.
Atleast it would have made the Greeks realise that not paying their taxes leads to a failed state.