I'm confused as to what you want here.
You say you want not every new champion to consist of an "extreme attempt to try something new". The reality is that every "simple" niche in the game is completely filled at this point with multiple competing champions, so the only way for Riot to bring something new to the game (which is what new champion releases are supposed to do anyways tbh) is to make complicated champions. It's not anything to do with Riot not liking simple champions; there simply isn't anything new a simple champion can bring to the game anymore.
Compounding this, of course, is the simple fact that Riot needs to make money. Most "simple" champions that are trumpeted as examples of the best kits of the "good old days" are boring as shit. See Taric, Ryze, Garen, and old Raka for examples of what I'm talking about. Very, very few players will pick Taric when he's not overpowered because he's simply no fun to play because of the lack of interesting gameplay.
That's not to say that simple champions can't be fun, because they can be. Take Ezreal, for example. Every spell in his kit is easy to grasp, so he's "simple", but he also creates fun gameplay through his skillshots. For another example, consider Ahri. Her kit is another very simple kit where each ability has an easily grasped effect and purpose, yet she possesses a surprising amount of depth.
The easy champions that are being reworked are invariably the ones that have play patterns that, although easy to learn, have no variety or interesting gameplay at all. Consider Ryze, for example. In the laning phase, his gameplan is to farm with tear/catalyst/shroud rush for infinite sustain and his snare to cockblock any attempt to actually kill him. In teamfights, he rolls his face on the keyboard while being incredibly bulky. It shouldn't take much to see that there's very little actually going on in his kit-no decisions or gameplay that other champions don't also offer while offering their own unique decisions.
On top of this, simple champions with straightforward, no nonsense kits are often very difficult to balance properly, as well. Consider Ryze again-he will build the same items in the same order almost regardless of his lane opponent. After his first back, he will harass the hell out of any melee laner with his low cooldown Q. If the melee ever blows their gapcloser to approach, snare, step back, and cast more Qs. Because there are no subjective decisions or even aiming for Ryze to do that other champions don't have to do more of, Ryze's laning phase is always either oppressive or terrible depending on his early base damages. If Q/E deal too much damage, it's impossible to lane against him as a melee; if they deal too little, Ryze is useless. There's no in between, either he harasses you to death or he deals more damage to your fingers than the enemy.
With complex champions like Gnar, Bard, Kallista, etc, there are definitely more difficult mechanics for new players to grasp. However, they also have many more levers by which they can be balanced. There are a thousand things they can toy around with on Kalista if she proves to be too strong. With Ryze, it's just his damage values, mana costs, and CDs.
So before I add more on to this incredibly long post, I'd like to hear more from you. What benefits do you see from creating more simple champions when there are already plenty of easily grasped champions for new players? What is so engaging about the gameplay of brainless champions like Ryze and Garen that makes them worth protecting instead of reworking them into a form that asks the player to make decisions? And where do you draw the line between "simple" and complex champions?
was the HARDEST champion for me to just pick up (
was two seasons away from existing). You have to use his spells in just the right order in order to get the most out of his resets. He's simple in concept, but that doesn't really make him "easy to use."
Conversely, you have a champion like 
. She has a ton of different effects, mostly tied to her mantra. But for me, even as a newer player, she felt intuitive, fun to use, and easy to pick up.
"too easy to play so he's not allowed to be strong"
