Let’s look at the past 40 years. Spain and Portugal have obviously gained immensely from membership. Sweden and Finland are better positioned inside the union than Norway which remained outside and became a rule-taker (implements EU law but gets no say). Former Soviet states like Estonia are very happy to be part of the West European community. Etc.

If you say “Greece,” then you’ll have to point to concrete evidence that the country would have somehow fared better outside the EU because it was such a basket case all along.