Getting to semi finals, finals or later cup rounds shouldnt be completely disregarded. Doing that implies that it is completely irrelevant if you go out in the 3rd round or the semi final which it isnt, the more you get to the later rounds of cup competitions the better team you are, the more likely better players or to stay/attract better players and the more chance you have of winning something. Very few teams go out early on most times and win something out of nowhere.
Where it becomes slightly less meaningful is like it was with Roberto when the league position was really poor and there were so many things wrong with what he was doing with us over a 2 year period that meant becoming a consistently successful team was highly unlikely. And some of those elements all came together for us to fall apart in the 2nd leg of the semi final at city.
The league is the main focus for every team because the better you in the league consistently season after season means the more chance you have of winning trophies and not just a one off trophy. If things pick up under Koeman, rectifies some of the errors and we get better and we finish well in the league for the next 2 seasons but go nowhere in the cups he will be questioned more about his future than if we finish well in the league for 2 seasons and get to say 2-3 semi finals in cups because in the second scenario it is showing all round consistency and you could see with some improvements trophies might not be far away.