It sounds like you're changing your stance that Harden had no other way to get to the Clippers
One reason to not take the word of a GM that he'll trade you in the future is that it is difficult to know what your current team will get in return for you. The GM works for the president/owner of the team and his #1 priority will always be to do what it best for the team over what is best for the outgoing player. If what is available for the outgoing player is far less valuable than anticipated, I'd argue the GM has a duty to not make a trade that is pennies on the dollar.
This gets back to my original point: If your #1 priority is going to a specific team and/or not playing for your current team moving forward, I would say that re-signing, a non-sign & trade deal would be about as foolish as it gets; it's the ONLY outcome that relinquishes your ability to obtain your #1 priority. Now if you value money more and are willing to roll the dice that other teams think you're worth significant assets, then that's fine, but know you're taking a risk.
Pretty simple stuff.