All EU countries have to publicly commit to joining the Euro and doing the work to do so. They are therefore required to join it, under any reasonable interpretation of the treaty language. The fact that some countries realized they could just not do what they agreed to and/or hack their economy to avoid the entry criteria, without any consequences, is good evidence that the treaties are indeed meaningless. But lax enforcement isn't the same as no requirement. The EU Commission could change their stance at any time.