You are talking about the windows store, right?

Correct me if I am wrong, but the Windows Store has been around since Windows 8, so its hardly new. Not as filled with millions of Apps like the iOS and Android Store, yes.

And certainly, Windows 10 Universal Apps are new, don't know if there is a special part of the store that highlights these, you might have some months of having a better chance for visibility until more and more people start creating universal apps.

On the other hand, neither the Windows Apps, not Windows Phone was a blazing success until now. I am not sure Windows 10 will do much to change that, no matter how good Windows 10 might be as a Desktop OS.

You really want to gamble supporting a mobile OS only about 2% of devices uses (number made up, but its in this region), and a part of the Desktop OS treated like the unwanted stepchild by most people owning said OS?

Yes, it is generally a good idea to increase your potential customer base to include the maximum number of potential customers.

With the cross-platform engines and tools out there it is quite common to build games that run on Android, iOS, Windows Phone, Blackberry, and more besides. New platforms are supported as soon as the cross-platform engine grows to it.

If you are not working with a cross-platform engine, then you are right, the reduced competition may be a reason to target the device. However, the reduced number of potential customers is a reason against targeting the device seems to far outweigh that.

Right now according to various sources, Windows Phone (all editions) makes up about 2.5% of the market. Android is currently dominating the analytics with just over half the market share (59.6%), and iOS has been slowly dropping and is currently 32.2%. Older devices with Java ME are about 4% of the market, BlackBerry devices are down to 1.4%.

However, when it comes to paying for apps, iOS users are still more likely to pay for apps than Android users, and the per-platform revenue stats are still nearly equal between the two, both getting around 45% of the money.

Choosing to only support Windows Phone 10 means you are targeting about 1% of the mobile market. The other 99% of the mobile market won't care about your game, even if you advertise it heavily. If you use a cross-platform library or a web interface, you can nearly all the market including Windows 10 phones.

It depends on how much work will be involved in porting your app to Win10. Depending on your framework, it could be as little as changing the dropdown to build UWP instead of for Android or whatever. In that case, I'd go for it.