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.