PM I'm working with "They are big enough to steal what we show them and beat us to market"
My response: "They're big enough to be paralyzed by the innovator's dilemma - so if we are executing well they will be unable to pivot and execute fast enough to keep up with us."
The key part there was "if we are executing well"
It's more likely that you will kill yourself through poor execution than the big players in your space will come and copy+crush you.