This is only true for sellers who would otherwise offer their own international shipping - many, many sellers don't, either because they've been burned enough times or they were too nervous ever to start. For them, every sale is "more sales" than "none".
And, reports of less international sales are definitely overblown. Until end of 2013, I did all my own international shipping on eBay. My international sales dropped at about the same time the very large price hike for international hit from USPS in January 2014. When I started with the GSP, I was (and still am) getting nearly as many international sales, with no more hassle than shipping domestically.
eBay is a dicey environment at times - my comfort level in offering international shipping through, say, Bricklink is far higher than it is through eBay. I am well able to handle my own international, but GSP means that in eBay's environment I don't have to. Other sellers may feel perfectly fine about selling internationally via eBay, and of course those folks shouldn't go with GSP.