There is absolutely no way that it will be more profitable to not port an already developed game.

Let's assume a game costs $30M to develop. It probably costs less than $5M to port it. If the publisher gets $20 a pop, how many sales does it take to cover $30M? 1.5M. Now, how many sales does it take to cover $35M? 1.75M? So, in order for the port to be profitable it has to sell at least 250K copies, which is 1/6 of what an exclusive would need to sell in order to break even.

Basically, if you have an already finished product and the costs to port it are so small, sales of the ported version would have to be ridiculously low for the port to not be profitable. I don't think there's any way getting a cut of used sales would overcome all of that.