You can change the shape of the triangle, but you can't grow it. The secret thing that makes a triangle larger or smaller? Money. Microsoft's triangle is a lot bigger than Joe Startup's triangle. They're both still triangles.
So when you're a startup and you need to build a lot without a lot of money, oh and fast, please, then something's going to give. This is when the word "no" starts to come up.
But I rarely actually say, "no." Because "no" really means "never." It's pretty rare that someone comes to me with something that is technically infeasible, so "never" isn't really the right answer. Rather, I wind up saying, "not yet" or "sure, but you'll have to not do this other thing."
And that's okay. That's not me being obstructionist. That's me looking at my fixed triangle points - usually resources and sometimes time - and saying, "here's how big functionality can be". I'd rather everyone know up front that there are constraints than tell a client, "we'll try" and not finish the entire wish list.
It's not "no". It's "not yet".