Why bother being a support?

So it's mid-late game and I see the enemy Master Yi coming towards our base on a ward. I, my adc (both at full build) and our top lane are just outside the base preparing to kill this yi who is isolated. Then he alpha strikes past us, kills the adc in a fraction of a second, and leaves before either of us can do anything.

Please understand, this is just an example. Half of you will want to tell me "you should have built frozen heart" and "obviously you should have been playing ad soraka (I picked one at random) to counter yi" but Yi isn't the point (not specifically anyway)

The point is, in a game full of assassins, fighters, and mages who can kill squishies without a second thought and without any interaction, why bother being a support anyway? The number of champions that can kill a squishy target with no warning, no interaction, and no counterplay is frankly ridiculous and is, in my opinion, hurting the game as a whole.

I understand that there needs to be a class to counter hypercarries and the like, but I don't think that the way it is being handled now is healthy. Assassins should have as much interaction as any other class. (to clarify: I understand that there are assassins that are much healthier and have some interaction to their kills. I am generalizing "assassins" as the champions that don't give any warning or chance to react before they kill someone: kha, rengar, talon, zed, kayne to some extent, etc.)

The argument could be made that, since they need to have enough damage to 100-0 their target to be relevant they are balanced by this threshold of usefulness (as in, they are an all or nothing contribution to the team) but this is a flawed argument for two reasons. First, they will always have enough damage when everyone is at full build. Second, the fact that this is the only source of balance for these champions speaks to their unhealthy positioning as non-interactive champions.

To make myself clear, this is not a rant about rengar being broken or kha needing to a nerf. The problem is not a power imbalance, but rather a lack of interactivity. These assassins that I am mentioning are unhealthy and frankly, no fun to play against. All the team can do is wait and hope that they don't have enough damage to kill the adc, and the adc just gets to stare at another grey screen. The assassins in question are always desperately struggling to have enough damage to stay relevant, and often die after killing their target. I don't believe that having two classes whose only job is to kill each other (in a boring repetitive combo) and then watch the rest of the fight, is a good thing for the enjoyment of this game. (It certainly isn't for me, and i've played both sides of this issue extensively)

I don't know how to fix it, but I really believe that something needs to be done at a fundamental level to make assassins more fun to play with and against.

Thank you for your time.