Key thing here is "3rd Party sales". In the UK, at least, you tend to go to a third party store BECAUSE you're looking for something that ISN'T 40k - otherwise you'd just go to GW, since there is pretty much a GW in most towns with a population over 20,000.
So yes, if you consider just the sales from FLGS without factoring in GW store sales, then it might appear that other systems are closing the gap. However once you factor in GW store sales then everyone else is still left behind in the dust.
However having played X-Wing, Warmahordes and GW, I can say from my point of view that GW is running on two things - Market Share and the best multipart plastic miniatures money can buy (if not the cheapest).
Other people do better resin, do decent metal, do decent plastics, but at the end of the day I have yet to see anything from anyone that matches the consistent quality being produced by GW on a grand scale. Also, if you ask anyone between 12-30 what toy soldiers they used to play with when they were younger, the odds are good that it was 40k/WFB. Hence the market share.
However in terms of rules, and bang for buck, GW is fast dropping in both, which has led to my selling off everything GW related I own apart from my two main armies, in order to invest more into X-Wing, which has a far richer and stronger rules system.