First of all, no, it is objective fact that a mechanic that is consistent is more competitive than one that is random. That's not to say that competition can't exist with rng, see poker. But there is simply no argument that a mechanic that is repeatable will be more competitive, assuming equal complexity. If you want to beat someone, you have to exploit the mechanic better than someone else. There's no way around that. You can't bank on the RNG spitting out what you want at the right time, or spitting out the result that you tend to play better with.

Sure you can argue if consistent is good for the game, but then you have to talk about what League is. it's a competitive game. MOBAs are popular because they are skill based, so anything that serves to compromise that, such as RNG, is going to be a bad thing.

The mistake is that people think "oh, we want more fresh and dynamic games, and RNG does that." But if you look at poker, a game that revolves entirely around RNG, it's not 'fresh and dynamic.' It just comes down to either reading a bluff, or long term bank roll management. The core thing that both have in common is decision making, and as long as there is interesting decision making, the resulting gameplay is interesting.

The reason plants are bad is that they don't lend any more interesting decision making, and that if more interesting decision making is what you want it can be done with the existing smite buff mechanic that already exists without adding an RNG mechanic that directly compromises the competitive aspect of League which is heavily skill based.

If FIFA was trying to make soccer more popular in America, they probably wouldn't add a mechanic where "every few minutes a spot on the field lights up, and in 30 seconds a big rock will pop up from the bottom of the field." Yes, it will change how players make their decisions, and it gives you some time for reactive decision making - just like plants, but it's fucking stupid because, again, it directly compromises the competitive aspect of the game.