First of all, it is currently a standard practice in a number of European competitions.
Yeah, but except the champion and a Cup runner-up, the best 3 or 4 clubs from the first league goes to European club competitions...
That doesn't really matter. The point is that when a team gains eligibility through both the Cup and the league, it is the next best team in the Cup that is also qualified, not the next best team in the league.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."