I agree with this, and this is one of the reasons why I think making the bot lane marksman the de facto VIP on every team is terrible for player choice. There are actually several more classes besides assassins who want to focus the VIP: divers want to go for a back liner and mess them up, vanguards want to lock down whoever they perceive as the largest threat, and mages want to blow up whoever's causing the most trouble, but on the flipside we also have supports choosing which member of their team whose power they want to amplify, and wardens figuring out which ally it would be most worth defending. A huge part of the decision-making process for most classes is conditional upon them picking priority targets to focus, so when the number of valid choices boils down to just one, a huge amount of depth is lost in the process. As mentioned above, there should be many more potentially valid carries besides the marksman, but even the support could themselves be a valid target, as they can power up and sustain their entire team (which I think counts as carrying in its own right).
As mentioned above as well, I think this also leads into the fact that, whenever the marksman is always the priority target on either team, this hurts marksman players as well. While some may like being the center of attention, I don't think anybody enjoys getting ganged up on constantly throughout an entire match. I don't think it's really fair to have to deal with getting hard-focused by multiple members of the enemy team at a time, all the time, even if there are also allies on your own team trying to protect you. While there should definitely be some games where the marksman pops off and becomes their team's MVP, that shouldn't happen every game, nor should the game force that situation to happen via stark differences in scaling and contribution.
In fact, even in the current framework where marksmen are essentially the only carry class around, this should still be an environment where the MVP changes as the game progresses. If invades or ganks happen early on, it might be worth taking out the fighter or tank first, since they'd often be capable of hurting the most while not yet getting all that tanky (this should also be one more reason why top lane should be afforded more agency over the other lanes early in the game). If teams are collecting in the mid game around objectives, prioritizing the mage or assassin when they're at the peak of their power could remove the biggest asset out of a fight. Afterwards, as games get to a close, focusing the marksman or the support powering them should be equally valid, as they're two halves of either team's carrying core. There's a ton of potential for more interesting tactical and strategic decision-making in League, some of which I don't think would require a huge overhaul to implement, and I think working towards that would be preferable to engineering a game state where fights increasingly rely on twitch mechanical plays in the very first few moments of combat, and otherwise have a fairly predictable and unchanging target selection paradigm.
main and Talon is not fun right now. I know you ADC players will say, "as soon as your ass gets 
you will hop over a wall and one shot us. After that you will to stalk us the ENTIRE game." Yes this is true, but this type of gameplay is really not that fun for us either.
. This is not fun for me as an assassin main and is REALLY not fun for ADC mains.
and a 1/3 
your gnna go after kog? If so what changes did Riot make to force u to make this decision?
