They're not average at defending. They play far more advanced than most full backs which is why they can occasionally be caught out of position (far less than they should be really). It's impossible for any defender to be high up the pitch and suddenly teleport back to stop a counter attack. No top attacking full back will be capable of doing that I'm afraid. If you want a more traditional full back who stays back, AWB is more similar to that, but these full backs are being discarded by top clubs and rightfully so because they are limited (and in AWB's case not even positionally very good). TAA, James and Cancelo all offer far more to their teams than any of the best defensive full backs will. For ETH's style of football, where he wants as many players to help the attack as possible, Hakimi would be perfect. Ultimately, it probably won't matter as it seems highly unlikely PSG would consider selling him.