New placements are complete and utter garbage

I did my placements for the preseason and went 8-2, pretty good. I ended the season g2 after climbing several tiers in the last few tiers from g4 in a week and couldnt quite get back into plat (i decayed from there after taking a break). Based on my placements i got placed in g1, witha bunch of diamonds and high plat players. At first I though "I'm utterly f'd" but soon realized it a great learning experience i climbed up to 95 lp almost at plat promos. Then ranked was unavailable... I thought oh well i can wait. Then they were gonna redo placements based on one game. "sure whatever matchw ill be against plats and whatnot and ill have to climb to get to promos again or if i win i might just get plat". We lost the game, toxic lower gold players and 2 teammates who had no idea how to play their champions (duskblade lee sin). I got placed in silver 3. How is that even relevant or possible? I lost almost 700 mmr in one game. My preseason record was even positive (17-8 or 18-7). I don't understand how with the number of games I had in preseason and climbing that I was placed 7 tiers below based on one loss.