It's already nearly impossible to do 51% attack on BTC, only if certain pools will fuse together.
Actually that is one of the biggest arguments IN FAVOR of a change to how mining works. The whole Idea of decentralization was to make Bitcoin safe from attacks and manipulation. Ideally your statement should have been "It is impossible to do a 51% attack even if ALL the big pools fuse together".
We are as far from that state as even possible. The very reason for this is that bitcoin mining encourages pooling hashing power together as tightly as possible. That leaves just two options: Join the biggest pool or invest in A LOT of hardware. The way things progress the future will be this: Single persons will stop mining alltogether. Mining and thus handling and veryfiing transactions will be done by a handful of big players that invested millions into hardware and that WILL enforce hefty fees for transactions to keep their businesses afloat. The network in practice will not be decentralized anymore.
