2 years is a long time in football, you never know we may even have CL football by then. Maybe Barca will be struggling financially and unable to trigger the clause. Maybe Mina will have an injury so they won't want to. Maybe they have 2 world class defenders already playing well and don't need another or Mina doesn't want to go there knowing he won't get first team football?
I really don't see this buy back clause as being as bad as some think it is. Just the media finding something to dig at Everton for because unUnited didn't get him.