Well, adopting the € is not optional. UK had a special deal to keep the £ back then (so did Danemark), when it was still possible, but this isn’t an option anymore. From the moment your economy meets the requirements, the country shall adopt the €. Czech and Sweden are overdue for the € adoption, they are dragging their feet. I’m not saying the unified currency is always in the best interest of each individual member country, but that’s part of the deal.