The point being that it doesn't matter if you believe people will not play the game because game performance; if they really like it the will ignore it. And if they paid for your game you should at least try to give them an experience as good as possible.
Yeah, in the perfect world all your buyers will have them. In real life sometimes don't.
Plus there is a lot of things you don't know; maybe he haves the minimum requirements but he is running a lot of background process because he installed a bunch of things he doesn't use.
If the game is so bogged down that the fixed timestep cycle is slowing down the game is broken for so many other reasons. Objects will start tunneling through other objects.