you cannot displace so much mass, and then just reintroduce it rapidly with no effected strain on the container.
If your theory "held water" (hahahahaha)....
then why does a container crack, split or break when filled to the brim with water and then capped and put in a freezer?
The water expands and breaks the container.
the effect of displacement when ice in water melts is ...water levels "rise" ...they don't stay constant as your theory postulates ...or are all the scientists saying that sea levels are rising due to melting icecaps, glaciers, etc...completely wrong in their findings?

There are always consequences.
The potential for unknown consequences is beyond "high" ...here.

The scientists who originally forecast the speed of climate change had to completely revamp their calculations, since the rate of melt has increased exponentially and they couldn't understand why, until they just happened to put two and two together concerning the melting permafrost planet wide ...and the release of all the trapped methane within, accelerating the entirety of the process exponentially.
Feeding the rate of speed of climate change.
They weren't counting on that.

There is a lot that folks aren't counting on...aren't even considering ...
Right now..the rate of acceleration is exponential...

Just wait and see what comes next.