If ones leaves the euro, it does not matter if ones has a primary surplus or not. If people flat out refuse to sell food and energy to you. This could even be disguised as punitive levy to recover the debt (which would have to be restructured when a country would leave the Euro and going into massive devaluation). Border controls (even refusing to buy your stuff killing the surplus), etc.

Whilst you are probably thinking the economics right, you are forgetting the (more important) political dimension of it: if a small country leaves the Euro there will be strong political forces trying to force that case to be a failure.

Short of autarky, leaving the Euro for a small country would be a massive gamble (probably a suicidal one).