Well, I think you can't create a mining council with powers over part of Bitcoin Network without the consent of all/most Bitcoin "stakeholders"/participants, otherwise Bitcoin would easily be attacked and conquered through the backdoor. I think they should derive whatever power they need from the majority of Bitcoin citizens and "stakeholders"... by citizens I mean people who strongly believe in its ideals/principles and uphold it them.
Technically, however, nothing is holding them back.
Would they destroy bitcoin's decentralisation? Yes
Would they decrease trust in our ecosystem? Probably, Yes
Would it be fair? No
..
Would it be technically possible to create a council with most of the biggest miners and unilaterally decide what they'll mine and what not? Yup...
Now, when i said "technically, nothing is holding them back", i do mean TECHNICALLY... What should hold them back is the fact that by doing this, they'll probably cause a massive dump in the price, a massive dip in trust, a big fight in the ecosystem, and they have little to gain from this (at least, at this point, and from the point of view of commercial mining farms.....).
One last point, when you say (and i quote) "otherwise Bitcoin would easily be attacked and conquered through the backdoor", you have to realise that it's not a back door... It's a heavily fortified gate, with a big waterway and a drawbridge, and heavily armed guards in the watchtower.... There is no "easy" backdoor... It's not like you and me could come together, form a council and disrupt our ecosystem... Your "ticket" to the council would have to be a decent share in the current hashrate... I mean, in order to pull this off, you'd need a combined mining farm that equals or exceeds 51% of the current mining capacity... It would be a billion dollar operation just to get a say in a council like this (unless you're running a pool, and you're using your pool participant's hashrate as a bargaining chip, in which case i hope 99% of your pool participants leave your pool for good).
We're not talking about home miners, not even about guys that run a couple dozen ASIC's... We're talking about huge companies that invested tens and tens of millions of dollars into ASIC's, and pool operators that have hundreds and hundreds of miners... If they want to, they do have the power to disrupt things, and they don't need our permission to do so i'm afraid.