I think that people who manage to achieve "impossible" tasks tend to be overly optimistic (and naïve) at first, consciously or not, this is a good trick to fuel their own motivation and that of others.
On the contrary, knowing/talking too much about the difficulties can quickly kill the fun and motivation.
He should talk about how great the end-goal will be and why it is important.
Would you rather end-users continue to get stuck with applications that ignore important things like accessibility, that can block some people from getting or keeping a job, so as not to kill some developers' motivation? I'm glad Raph is talking about the difficulties so they (hopefully) won't be ignored or clumsily bolted on afterward this time.
Edit to add:
"Those who cannot learn from history are doomed to repeat it." --George Santayana
I think the time for recklessly moving fast and breaking things in software, without taking into account known complexity and avoiding the mistakes of the past, is over. Our impact on the world, and the resulting responsibility, is simply too great for that.
On the contrary, knowing/talking too much about the difficulties can quickly kill the fun and motivation.
He should talk about how great the end-goal will be and why it is important.