How you do in your matches is not taken into account in any way. The only thing that matters is your MMR compared to the average of the division you're in.
If you won 6/10 and got Bronze 5, might I assume that you were somewhere between Bronze 5 and 2 last season? The only time that your Placement matches entirely determine your placement is when you've never been ranked before; if you have, it's based on the MMR you last had minus some for the soft reset (this is new for this season and on).
On average, it appears most people dropped 3 Divisions coming into the new season(the max placement is Plat, so Challengers were even dropped 3 Tiers). The only way to come out equal or ahead into this season, is if your MMR was a lot higher than where you were sitting before.