Honestly, there have been many times when a certain team is "nailed on" for the league, only to lose it later.
It is still only December. There is still half a season left to play.
The PL is a very competitive league and we have seen many times now that a single defeat can often send top teams into a downward spiral, and a "blip" by any team could easily result in 10-15 dropped points within a very short period of time.
There is absolutely no guarantee that the league will be a two-horse race between Chelsea and Liverpool. Especially when both those teams still have to come to Old Trafford.