Late to this topic, but I'll reply anyway.
Personally, I still think IL-2 Sturmovik: 1946 is a great WWII combat sim. Not sure why you don't like Steam, but if you get it from there, it should install on your Windows 10 machine without any problems. It can also be modded using various community-created add-ons, but that takes some patience and following the mod install instructions to the letter.
IL-2 Sturmovik: Cliffs of Dover has better graphics than 1946, but I found the AI wasn't as good to dogfight against ... they seemed rather dumb compared to the AI in 1946. I could just be imagining that, but others I flew with in multiplayer coop thought the same thing.
DCS World is an amazing (and fully up-to-date with modern PC hardware and Windows 10) air combat simulation that is free to download, but you'll have to buy the decent warbirds like the Spitfire, FW-190 D-9 Dora, and the P-51D Mustang. You can download the base game for free from their official website or get it on Steam (that is, if you have a change of heart about them), but like I say, you have to buy the *good* versions of the warbirds if you want to fly them. You can also buy WWII air combat campaigns and maps (they have a WWII Normandy map) if you want the fully immersive experience. If you have any difficulties getting your bearings, check out the video tutorials on the Grim Reapers YouTube channel (here's some Spitfire tutorials) or, if videos aren't your thing, there's always the large and extremely helpful forum community at forums.eagle.ru.
I'm currently enjoying the hell out of DCS World. Here's a little video montage I made recently of a Spitfire mission I flew with one of my fellow CSIM forum buddies (warning: harsh language and non-period rock and roll music):