So i did my 10 placement games
How many of those did you actually win?
went from S5 to B5
If after you got to S5 last season you stayed there for a while and had to play a bunch of games in S5 you were losing enough games in S5 that your MMR actually dropped to somewhere in Bronze. If you did well on your placements and dropped you might have already had a mid/low Bronze MMR.
if people are going to get screwed a the beginning of every season
I ended Season 4 freshly demoted to Bronze 4. I made it to Silver 4 during preseason holding about a 70% winrate after the season ended. I won 7/10 of my provisionals this season and I got placed in Silver 5. I am getting over 20lp for a win atm so I don't see how the system isn't fair. I held the same winrate as I did during preseason and therefore got placed where I was before.