>
> But the majority of the hashrate can now perform double spends on your
> chain! They can send bitcoins to exchanges, sell it, extract the money and
> build a new longer chain to get their bitcoins back.
>
Obviously if the majority of the mining hash rate is doing double spending
attacks on exchanges then the Bitcoin experiment is resolved as a failure
and it will become abandoned. This has been known since day one: it's in
the white paper. The basic assumption behind Bitcoin is that only a
minority of actors are dishonest - if the majority are then Satoshi's
scheme does not work.

So you are not stating anything new here.