Personally, I feel "opportunism" in assassins has mostly ended up translating to waiting until an enemy separates from their team -- which, I agree, is precisely what pushes them out of organized play, as they don't function at all when teams stay grouped. However, I think the larger problem is that League's meta overall has reduced itself to just grouping up nonstop past a certain point in the game -- splitpushing isn't an effective strategy, because Riot has actively balanced it out of competitive play, and generally the only way to respond to numbers is with equal numbers, and ideally equal power, since that's what steamrolls objectives. Ideally, League's late-game should allow for splitpushing in a healthy way, because that would allow assassins to thrive by picking those opportunities and hunting down splitpushers.
Beyond that, though, the problem with assassins in competitive play is that they simply get hard-countered by defense: in Seasons 3, 4, and 5, for example, assassins frequently dominated the competitive scene, not just because the framework for opportunism had changed, but because marksmen had far fewer overall defenses, even in the middle of their own team. This allowed assassins like Zed, LeBlanc, Fizz, Ahri, etc. to make picks even against grouped-up teams, because their attack really boiled down to a duel against their target. Nowadays, those kinds of plays are no longer possible when assassins have more delayed damage, and even a second of forewarning is already enough for the marksman to receive immense amounts of healing, shielding, damage reduction, various unique types of protection from certain wardens, as well as crowd control from their support and tank. Defense is far too easy to apply relative to offense, and it is this imbalance that has pushed assassins out of pro play as well.
Just to be clear, I think it's healthy for assassins to spend more time killing their targets, assuming it's done in a manner that doesn't feel forced (LeBlanc's reworked passive was infamous for this), and in a perfect environment, assassins should be giving their opponent more time to react and still have a chance of succeeding. It is the state of defense in the game that is problematic, especially when applied to marksmen, and it's combined particularly poorly with the assassin reworks to drive the class out of competitive viability (but nowhere else, since other divisions give more opportunities to assassins). If Riot were to address this problem, namely by making protection less abundant and less reliable overall, it would give assassins plenty more room to maneuver even at the highest levels of play, as they wouldn't have to kill a target that is currently guaranteed the full protection of their whole team every high-level, organized game.