Even due to miners' consortium they may work together and may decide things like corporate but all will be possible until they face another competitor. I mean it is highly possible another big players to be joining bitcoin mining to compete against the current players. So, their unite will not work for them forever.
Miners merging causes centralization. When miners merge and form corporations, we are back in a centralized banking system and there is no new innovation going on there. Mind as well use Paypal or your web bank. They are probably also much faster than Bitcoin. A dynamic blocksize prevents miners from merging because they have no benefit from doing that.