Now, I haven't even finished my placements yet, but...

...why do you encourage progression then move people down 3-5 Divisions at the start of the next season?

For every Season I've played Ranked (Seasons 3-5), I've steadily improved. Bronze III to Silver III, Silver III to Gold III, Gold III to Platinum III. This is what I aim for every season because I'm simply not the type of person to spend 10+ hours a day playing ranked. It's simply not healthy for me - physically, mentally and academically.

If Riot dropped me to Gold III off the back of placements, I'd be forced to re-complete 100% of my efforts in Season 5 before I even begin my Season 6 goal of Diamond III. The minimum I've seen someone dropped is Plat III to Plat V and that was with 7-3 placements.