Why this game has gone downhill

Because people kept crying about how unfun it was to play against things that you had to pre-emptively defend from, or strategise against, rather than playing reactively against. Examples - Rengar ultimate, Fiddlesticks ultimate, Jungling (as in how it used to be way back before vision was all over the place or plants existed) and so on. It used to be that if the enemy team played an assassin with strong roams you'd have to play a certain way, people cried about it, so they nerfed assassins It used to be that if you wanted to defend from junglers you had to invest in vision, people cried about it, so Riot removed green wards. It used to be that if you wanted to have good lane access as a jungler you'd pick someone with high mobility, and if you wanted good teamfight you'd take a pick that sacrificed that ward hoppin wall jumping mobility for teamfight presence. People cried, so Riot introduced crap like blast plants - now every jungler has the gank paths previously reserved for the most mobile avaialble to them. Same thing with honey plant and sustain in jungle. Same thing with crab and vision control Lane bullies used to be more of a thing, that way you could stop shit from scaling but people cried sooo... There were these two classes, assassins and bruisers, who were really good at punishing ranged champions who made the tactical mistake of positioning, or strategical mistake of going solo-dolo, but people cried sooo... There used to be this class who had an entire lane to themselves, but because people cried Riot gave them, or nerfed everything else, until they could respond reactively to everything, which pushed them over the top, made them grossly op, peolpe cried soo....

Long story short? It's all generalisation. The nuances of this game are slowly being dismantled.