The following is my interpretation only, and probably wrong.[:)]
First, you must understand the critical idea of doublethink in Orwell's writings. Doublethought is how the people hold two contradictory ideas in their head at the same time, by the suspension of rationality. War is peace, so to speak. Slavery is freedom.
The idea is that by the doctrine of IngSoc, freedom is translated into a freedom from thought and responsibility. The mental connection is made such that being free can only exist by creating imposed limits. Because Syme wishes to believe that, and cannot bear the reality of what he is doing, he does believe it, and so carries out his work. Does he really want this? The nature of the state is such that it dictates what the citizens want.
It all depends on what is better. The point of Ingsoc is control, control to such an extent that it extends to self-control within their brain. If a strict and rigid way of thinking is ingrain so far into the language of thought, then it is easier for absolute requirements to be ingrained into the mind. By starting from a new language, the erasure of history is complete, and the power of the state affirmed.