Having a squad of 'internationals' is no guarantee of success. Like club football some players gain caps not through being the best but by being the best suited to do the task the manager requires for the team. Conversely some outstanding players never gain caps for the same reason. It is a question of building a team best equipped to implement the coaches style of play. Koeman has just begun,it will take time. Traditionally ,barring disaster, a manager is allowed 3 seasons to get it right, Koeman is'nt even half way through his first. Some patience is required something you appear,at present,to signally lack.
Your view of defending seems deliberately naive. Defending is what the team does when not in possession, not just the back four. Successful clubs defend well as a whole team. In that regard we have definitely,in my opinion, improved.