At the begining of the season, there will likely be some sort of soft reset where your mmr is put midway between your mmr at that time and 1200. This means that any mmr you gain or lose now will instantly be half as valuable the second the next season starts. On top of that, each of your placement games will be worth a lot more than regular games (probably 4x the mmr gain/loss if riot keeps the same formula as season 1 and 2). This means that each of your games now are worth 1/8 of what your promotion games are worth. On top of that, most players aren't losing every single game even if they are playing for fun because they still have the skills which got them to their current elo to being with even if they aren't using all of it.
All of this means that even if someone trolls a ton of games in the pre-season, they can turn all of that around with just one or two placement games. Finally, even if someone were to completely mess up their placement, the ranked ladder is designed to get everyone to the skill bracket they belong over the long run. Everyone will has the entirety of next season to get back to where they belong.