i totally understand your point of view but i can tell you, its exactly why you are silver. you can do it, you can learn lee sin mechanics in 5 games to be as good as LCS players. trust me, trust your own abilities. ofcourse this is overexaggerated but its basically true.
the point where the combos get clunky is when things happen that you didnt plan ahead or anticipate. this can be as simple as the enemy champion just casually walking, because where he is NOW your combo would be fluent, but where he is in 0.5 seconds (which is about 200 range away), its allready kinda not what you planned. like when you see me do combos with lee sin or any champion really, you see me make so many mistakes that, if im not dead sick, come from failing to recognize the state of the game, not pressing wrong buttons at wrong times.
i mean, for me it goes like this: x-w-r-d. it doesnt have to be lightning fast because basically everything you can do combos with has animation times, and even if they are very short like 0.3 - 0.5 seconds, this is allready HUGE. like, your brain is SO FUCKING FAST, most people notice the difference between 20 and 30 MS ping. 0.3 seconds is a delay of 300 MS, that is ENDLESS for your brain. and pressing 4 buttons on your keyboard is the easiest thing in the world because it factors only 4 things your brain has to do at that exact time.
tho pressing those buttons RIGHT is where it gets problematic because you have to consider EVERYTHING IN TH GAME and thats way over hundreds and thousands of informations you dont just have to process, but you also have to HAVE at first. you have to HAVE a good understanding of the game and alot of experience to do it right.
tho to get back to what i was saying in the beginning, playing things that are problematic for you makes you better at the game. for example, if you have trouble with lee sin, play him alot. if you have trouble with positioning in fights and when to engage on what and keeping an eye on enemy abilities and cooldowns, probably play master yi. if you have trouble being invaded in your jungle and having no clue how to defend against it, probably play the weakest early/duelist junglers you can find. play fiddle, amumu or whatnot. and get your ass handed to you until your brain needs to develop some sort of strategy to avoid constant failure.
i mean, the less forgiving a champion is, the more your brain is forced to learn. if you play irelia, zed and nidalee every game, ofcourse you will suck forever. sure you can maybe become a diamondplayer when you are really good at dueling with zed, but you will still have no idea about how the game works and i wouldnt want you on my team. i'd rather have that yi player on my team who knows how to teamfight.