So apparently the core developers want to change bitcoin POW, I want to ask that when the majority of the bitcoin community is slowly reaching a consensus on BTU then instead of supporting the consensus why are they hell bent on destroying btc?
Is bitcoin no longer a decentralized system where everyone can decide for themselves and then the majority wins?
This is not the definition of a "decentralized system", in fact. You are confusing a decentralized system and a democratic (centralized) system.
A decentralized system is a system where there is no deciding power or hierarchy. "the majority wins" means that the minority must undergo the will of the majority. In a decentralized system, there are no votes, but there are
emergent properties nobody actually decided on, and certainly not a "majority". For instance, price setting in a competitive market is a decentralized system. Biological evolution is a decentralized system. Science is a decentralized system. The internet is partly a decentralized system.
Of course, de facto, there is some "weight of the majority", but that doesn't come about with voting, but rather as an emergent property. If the majority DOES X, then the consequences of X will lead to properties of the decentralized system.
From the moment that there is voting, governance and so on, the system is centralized. The dynamics is not emerging any more, but decided upon, and imposed.
As to your question regarding bitcoin: bitcoin is losing its decentralized aspect, but it isn't yet fully centralized. As long as there are more than 3 non-colliding miners of which none has more than 50%, centralization is not complete. For the moment, 5 miners are deciding on the fate of bitcoin. As long as they don't go and sit together in a room, bitcoin is still decentralized over a few entities. The day these 5 people talk to one another, bitcoin reaches the end of its decentralized part of history.
The principal need for decentralisation in bitcoin is immutability (of history and of protocol) and permissionlessness.
Which doesn't mean that bitcoin will not continue to function. It will be like facebook or google. It is almost there, but not yet. Immutability of history will remain. Protocol however, might change. And the most problematic aspect is most certainly permissionlessness. You will need your "licence to use bitcoin" eventually.