The conversation in this thread demonstrates why first-past-the-post voting is so hard to replace. There is no consensus on which alternative system to replace it with.
The real barrier is that a change to the voting system would have to be approved by a party that is winning under the current system. Convincing a party to agree to any change is much harder than picking one of the many good options.
Also, I hope you don't mind if I mention that your point is eerily analogous to a dictator saying "We can't introduce democracy, because then people would disagree on who the leader should be."
For example, if the Democrats were afraid that the Green party would split their vote, they might support RCV. Therefore, arguably, the best strategy for bringing about voting reform might be to vote for the Green party, even if you don't support any other Green party policies, or any Democrat policies.