But I think that there is also another point to make. It's true that there is often space to be the one idea that delivers in a way that is easy enough to use for the general user to the point of having the chance to be the winning product. But I believe that that kind of answers from investors shows in general a pretty clouded judgement.
I think that when they respond like that, it means that they don't actually like the idea in the first place.
Otherwise, exactly "because" there is some other similar product around, that in general means that there is some level of market validation. And if it's possible to show that those companies are profitable, then it's likely, if they trust your team, that also your company will be profitable.
This should be enough, you don't necessarily need to convince them that you will kill them all, you just need to convince them that you will make money.
Then, after you convinced them, you can keep going with your goal of being the unique winner of that market.