> it's just that if the value of the currency can be manipulated by small group, it is not removing the need for trust, just shifting it...

I think there's different aspects to "manipulated".

Large holders or exchanges can always manipulate the price, this is true for cryptocurrencies, the stock market and other things as well. Cryptocurrencies is however much less mature and more easily manipulated, but I foresee it to stabilize in the years to come.

If we're thinking of the miners to manipulate the coin I think the real genius of Bitcoin is they are heavily incentivized not to. All miners are heavily invested in Bitcoin's success so it's in their best interest not to destroy Bitcoin's value.

> I thought the point of decentralisation was to remove the need to trust others...

To be clear: what miners can do if they collude is try to revert transactions or block new transactions. If this happens it would be obvious and Bitcoin's value would be severely damaged.

There is trust yes, but it's more trust in the incentives behind Bitcoin than to specific miners themselves.