Currently, most mining power is controlled directly or indirectly by just one organization. This organization has the power to bully smaller miners into voting their way on issues that affect the Bitcoin network's rules, and it's about to become even more powerful:
Bitmain, one of bitcoin’s biggest miners with a current hash-share of around 15% of the network, is to launch a new giant mining facility this December with 45 rooms, independent substations, offices and a total size of 140,000 kilowatts.
By some calculations, that translates to around 45% of the network hash-share, but it is not clear whether the new facilities are simply for a relocation or to accommodate additional mining power.
Currently, mining Bitcoin is not practical for home users. With a PoW change, the opportunity for CPU & GPU mining of Bitcoin will again become available.
Once mining is centralized, censorship of "undesirable" transactions will become possible:
But Chinese power subsidizes mining all the same, and it doesn't just cause economic dislocation. It undercuts Bitcoin's security. A blockchain system maintained by entities within a single government's jurisdiction is at greater risk of political manipulation and censorship.
Changing the proof-of-work will fire the current crop of miners and open the door for quickly and easily scaling the network.