I'm just speaking generally. Sure, there's several anecdotal players that were diamond/master and are now silver. I think people underestimate preseason MMR. People start playing less seriously during preseason and tank their MMR so when reset rolls around they end up lower than their previous LP indicated.
Using my own anecdotal example, I ended the regular season in plat I, ended preseason with Diamond V. Started Season 6 plat V. Very fair if you ask me so I didn't jump on the boards and complain like some people. My roommate, ended the regular season in Diamond IV. Then he dicked around during preseason and tanked his MMR and started season 6 Gold III.
Then there were probably players in his games like "Wow, I'm gold and I'm playing with a guy with a diamond border. That must mean I'm diamond too." No, it means that there are still plat+ players climbing because they don't play a lot, tanked their MMR, started season late, etc. After this split of LCS, things will be more even and as they should be.
Rereading my post, it sounds more aggressive than I wanted it to be. Sorry about that lol.