Now that assassins get changed, the class they are supposed to counter needs changes too!
What is the job of an assassin? To take out high priority targets such as mages and marksmen. Mages usually have a significant amount of counterplay in their kits, as they are gated by cooldowns and mana and also have to hit thier abilities while often also being immobile as fuck. So they are not exactly a problem and if an issue with a specific champ arises it can be taken care of by an individual basis.
Marksmen however are a whole different story. I mean, it should be obvious, right? Since season 1 you have got an adc on every team. No match is played without them. They are the root of all damage creep. And now their hardest counter, the assassins got changed and with that they also got nerfed in the way that they are more healthy to play against, which in itself is not a bad thing. HOWEVER, this will have devastating effects on marksmen balance.
ALREADY we got them dominating our games with 2-3 in a team at times... and now you want to buff them even harder? You don't have to be exactly smart to realize that this is a very bad idea...
Also... Riot, have you ever thought that maybe, maybe people are sick of the same class of champions dominating for over half a decade? You talk about "spicing up the game" with plants and stuff, but you could just achieve this goal with knocking down marksmen from their #1 spot. The entire meta would break apart and sink in wonderful, wonderful chaos for several months. A time where everything and nothing is suddenly viable, all depending on the individual games and matchups. You want to keep this game fresh? How about kicking out the one thing that ha sbeen keeping it stale then?