He does have stamina problems, but Mourinho rated him, he was flabbergasted at Ancelotti's decision(it was his decision solely) to sell him, so were prominent players such as Ronaldo, Arbeloa, Benzema and others, the former especially was notoriously unhappy about Ozil's sale. At the end of the day, different managers have different views, and Ancelotti was one of the few who did not rate Ozil that highly.
Plus did you not watch Ozil at Real, the guy was amazing, he was their main creative outlet, and he raked a high number of assists every season(I believe the highest in Europe in the past 3 years), you just cannot erase that overnight because he isn't performing for Arsenal.
You can go on all day about how he's been disappointing at arsenal, and i would agree with you completely, because at the business end of the season, he has gone missing and is hurting us a lot. But that is not grounds for revisionism, because he was without any doubt world class for Real, and I doubt you were so sceptical of him in the past few years.