Something similar has already happened.
Examples: Islam and Mormonism. Examine each closely, and each resembles a Christian heresy.
Not to mention the fact that the Jews themselves consider Christianity to be a "blasphemous adulteration" of their religion. That aside from the fact that the Catholic Church displays syncretism as it absorbed the myriad faiths of the Roman Empire.
Protestantism, by and large, is a movement to go back to what one could call "original" Christianity.
“Christ would be added to Buddha as a god or guru, and Christianity would at best become another sect of Hinduism.”
Why would that be bad? Why are so many obsessed with the repudiation of the beliefs of others rather than with the wider acceptance of their own belief? Nobody is asking those who believe in Christ as the only savior to change their belief. How would it matter if Buddhists & Hindus accept the teachings of Christ but not necessarily the contention that no other path exists? Would they not be benefitted by the teachings of Christ? Is it anyone’s argument that the teachings have no intrinsic value by themselves and serve no purpose unless it is also accompanied by the total repudiation of other beliefs? Even if that be the argument, why impose it on Buddhists & Hindus who do not share that conviction?