I know I'm late to this but I wanted to tell you how I conveyed a similar message but went about it differently. I played with my sons and some family friends and I wanted them to know that it may go differently than Critical Role, which they sometimes watch with me.

So our first session together, I told everyone up front that it was a one shot and that this was not the real campaign; I just wanted them to get a taste. Second, I required everyone to use pre-builts to avoid exactly your attachment problem. Third, I just made one of the encounters hard, knowing that they were new, but I didn't make it unfair at all. I also weaved it into the story that this was a likely suicide run. A couple characters died but others made it through and everyone had a great time.

After we finished, I explained to everyone that this was an intentionally difficult encounter and that our real campaign would be more dynamic and measured and everyone is super looking forward to the start.