That's an irrelevant technicality. In practice, the countries can postpone the adoption of euro indefinitely, simply by refusing to provide evidence for meeting the economic requirements.

This is the strategy of Sweden, for instance, to retain the crown. If Sweden were to meet the criteria it would have to adopt the euro, but it never needs to show that it does meet the criteria, so it can keep its own monetary policy.

(I live in Finland, and while I think euro is personally convenient for me, it has been very bad for my country's economy).

Good point. UK & Denmark both have formal de jure opt outs, negotiated on launch of the EUR project. Sweden is a de facto opt out. By not entering the ERM trial period and managing SEK into a band with EUR, SE never enters the Eurozone.

The mismanagement of Nokia has been a much bigger problem for Finland's economy than the Euro ever was.

Hardly. Even if it is big, it is just one company. A country's economic policy cannot be based on expecting perfect performance from one company. Saying it is more significant than abandoning the possibility of own monetary policy sounds like full-scale whataboutism.