I used to have the same fixed mindset as you. However, it is possible to have a bad day, game or mood. I think that the system looks at the player as a whole not one snapshot of their skill. I started in Silver 5 in Season 3. Got "trolled" all the way down the bronze 5. Sure I was pissed at first but as someone with a long history of participating in sporting activities I knew it was impossible for me to get better, work my way up back to Silver and Gold by just sitting there and blaming someone else.
In short, there is always an aspect of the game that you can better in. Sure maybe you can consistently achieve 100 cs/ 10 min with any champ in any role. But what about your teamfighting skills? How about late game decision making? You can even get role specific. How often am I ganking? Where is the enemy jungler? Should I roam right now from mid? If you focus on your skill and continual improvement, you will climb the latter. Guaranteed.
Hopefully I've helped you in some way. If not, well good luck and Have fun at least.