G'day James,
I was talking about this exact issue with a friend of mine only a few days ago.
Call me a sentimentalist but fair dinkum, I know some people might take a walk down that road but I can't begin to fathom that someone might get a filly and put it down just because they wanted a colt. It is hard enough to get a live, healthy foal as it is, so much can go wrong that you don't have any control over. I can understand foals born with major defects going that way but to a large degree I expect that sort of thing already happens now.
My gut feeling is that as the POLF is increasingly adopted it's going to be increasingly reflected in/result in an overall 'on paper' drop in foal registrations because some of them will simply not make it that far whereas before this perhaps they were a bit more likely to have done so.
At the same time as the registrations decrease I don't think it will result in any significantly negative changes in the overall available to race horse population. The ones that are still considered to be good enough and are registered accordingly were pretty much always going to get there anyway, regardless.