Although there are still benefits in joining the EU the massive benefits like billions of Euros for internal development have gone.You will no longer see massive investment in infrastructure like the EU did in Greece. I remember in countries like Ireland the signs at the start of major building projects carrying Sponsored by the European Union written on them. Those days are long gone.
The major benefit for the current EU is that Turkey would be a buffer between the EU and the more radical Islamic countries. It is very difficult to understand the position of the current Turkish government. On the one hand they want to join thee EU and on the other hand they want closer ties with Russia and the Islamic states.
I can understand the French and German opposition in that Europe is being overrun with foreign migrants. Although Britain supports the Turkish application it would not be long before the complaints start that the Turks are taking all jobs. Similar to now but it is the Polish that have taken the jobs.