Hard to say, need to know some things and honestly some of them are estimations.
1. Cost to port to X1
2. Additional sales and profits to titles due to port
Essentially if 2 exceeds 1 by a certain factor it is not a mistake. If not, then it is.
The issue is defining these. The cost is likely not much as others have pointed out the similar architecture reduces costs quite a bit. The harder point to define is additional sales. Many people think the following. Game sold X on system A and Y on system B, therefore porting to system B added Y sales. The flaw in this thinking is that without the port the amount X on system A is guaranteed to be greater. The challenge is estimating what that X would have been and how it compares to X+Y.
The other talking point could be does porting expand a brand versus does it dilute it by not being as "special" to a particular base.
Honestly I am not sure, but if I can play games I want not bothered that people that own other systems can enjoy them as well.
