You have to be careful to make a distinction being in the EU and using the Euro as your currency. You can benefit from the EU common market, with its uniform rules/standards, easy capital flows, subsidies, and industrial policy. All without using the Euro as your currency, and being subject to the monetary policy of the ECB.
Being in the EU without using the Euro has been pretty good for Poland.
They are free to be. When joining the EU, Poland accepted to join the monetary union at some point, but there is no date set, so in theory it can be infinite. And with its current strategy to drastically modernize and increase its military force, there is no doubt it will not met the financial criteria anytime soon so nobody is worried about that.
Being in the EU without using the Euro has been pretty good for Poland.