Not really a mistake in your assumptions but in the context of the situation. It's no longer about currency substitution (eurodization in this case) , it's about a soon to be EU member that did not follow the criteria the EU deems important for price and growth stability (and further accession to eurozone).
That is - the EU is concerned about any deviations to the treaties. Notably the convergence criteria (or Maastricht criteria), particularly after the euro-debt crisis. So unlike normal currency substitution (dollarization for example) which has a strictly economy side, Montenegro eurodization has a political side.
This issue is not new. It started at about the same time as the talks for accession of Montenegro to the EU. You see, for a country to be inside the eurozone (which is different from adopting the euro), it needs to follow the convergence criteria, notably:
Agreed in Maastricht by the EU Member States in 1991 as part of the
preparations for introduction of the euro, the convergence criteria
are formally defined as a set of macroeconomic indicators which
- Price stability, to show inflation is controlled;
- Soundness and
sustainability of public finances, through limits on government
borrowing and national debt to avoid excessive deficit;
stability, through participation in the Exchange Rate Mechanism (ERM
II) for at least two years without strong deviations from the ERM II
- Long-term interest rates, to assess the durability of
the convergence achieved by fulfilling the other criteria
exchange-rate stability criterion is chosen to demonstrate that a
Member State can manage its economy without recourse to excessive
currency fluctuations, which mimics the conditions when the Member
State joins the euro area and its control of monetary policy passes to
the European Central Bank (ECB). It also provides an indication of the
appropriate conversion rate that should be applied when the Member
State qualifies and its currency is irrevocably fixed.
Since Montenegro is already using the euro you have a new precedent. I think it's very unlikely that the EU would require Montenegro to stop using the euro just to follow the treaty rules (it's more likely that Montenegro central bank would remain a kind of silent observer for a couple of years after accession, but this is very speculative).