Arbitration is a minor issue from a trade agreements point of view. The courts are not used that often, they usually side with states, and they're put in place mostly because neither country wants to give up its sovereignty to be at the mercy of the other country's courts. The natural solution is to make an a-national court to handle international disputes.
Politically, the issue has become very polarized, because even allowing an international court to settle disputes is seen as giving up sovereignty. Of course, I would love it if everyone settled in the court of my choosing, but it doesn't work that way. People also get concerned that these courts lack democratic accountability. But of course all courts lack democratic accountability...
I should add: here in the states, we worry that these courts are a backdoor for Canadian-conglomerate-friendly courts. They can't be simultaneously friendly to both the US and to Canada...