What is wrong with the goal of decentralizing development across multiple competing implementations?

There are no problems and it is in fact very healthy for a decentralized system. We wanted decentralization? Now we have it hard but those emotionally attached to Core are shitting their pants for overblown fear mongered reasons.
I absolutly agree with this! Having more options for alternative clients increases decentralization. This is a political necessity for the reosons of decentralization and freedom, whether you agree with Bitcoin XT or not.