Yes, you are right about Thresh and of course champion strength has something to do with the result, 4.20 Warwick demonstrated that. But what you said about Thresh along with who knows what else in that time period demonstrates the point. Thresh's statistic were changed and went down due to factors that had absolutely nothing to do with Thresh's intrinsic strength as a champion.
To demonstrate something logically and with scientific testing, variables have to be isolated and controlled for the results of the test to matter. If they aren't, the results are meaningless because we would not know what the results are actually indicative of. Win rates, pick rates, and everything else are results in which nothing has been controlled and it can not be reliably said what the results reflect, especially not numbers in a champion's kit.
Lee-sin and Skarner are two champions which most who follow these things know about, hence why I used them.
Lee-sin is famous, and famous two things if nothing else. 1: Lee-sin is blind. Many, many repetitive jokes are made about this. 2. Lee-sin is one of the best junglers. Period. He has been for years. Lee-sin and Jarvan are frequently named to be the perennial top two junglers. Everyone knows that Lee-sin is powerful, even too powerful. Riot acknowledges frequently that Lee-sin has been too powerful for far too long. And then there is Morello's now famous "Lee-sin just shits on people" quote. However, despite how strong he is, Lee-sin's win rates remain relatively low. He is people's one-word reminder about win-rates.
Skarner is a jungler who is on the other side of the spectrum to Lee-sin. He is not one of the few in the champion pool of viable junglers like Lee-sin is. Skarner's not really viable, not even really competitive. A champion among the lowest pick rates in the game, lower than Urgot, even lower than Galio. Skarner has long had problems and Riot has for a long time side basically said "We don't know how to make him work" even through all their reworks and retooling and the only way they have ever figured out to would have to be buff him to such a ridiculous extent he would have to be OP. Funny enough though, today's win rate for Skarner is 54.6%. His combined all region win rate has only dropped below 50% once in the past two weeks. Who knows why that is.