One of X-Planes strengths is also its weakness. They support Windows, Mac, and Linux, and on top of that it works quite well on even old hardware, if you accept you have to turn the detail down.
The XP code base sounds like it has a lot of baggage to carry forward, if you read the posts by Ben and JustSid, to maintain that compatibility.
MSFS has one platform to support, and one API, DX11, and hopefully DX12 some time next year.
TBH. I donāt expect much more performance with DX12. Maybe 10 FPS. Flightsims has always been hardware killer. DX12 has lower CPU but more GPU usage if I remeber correctly. Ok, there are also a lot of benefits like DLSS.
You might be surprised how much of code dating back to FS9 is still carried over into FS2020 though.
Thatās true, but they still only have to support Windows/DX11. That has to be easier than having to support Windows/OpenGL/Vulkan, Mac/OpenGL/Metal, and the many flavours of Linux/OpenGL/Vulkan.
Iād be glad to know as well!
The modern rendering engine part of it can be developed independently of the rest though, thus can be outsourced if needed but it looks like this is one core feature they usually do in-house.
If you look closely, the XP11 data model is already very good: OBJ files, navdata, weather data, etcā¦ These are all proven formats able to cope with the contingency of a modern engine. The core simulation components are also good: flight model, thrust model, etcā¦
So in essence, without any new features, it would just take rewriting the rendering engine and its shaders based on a single rendering model and a single rendering API. There are 4 at least in XP11 right now: (deferred + forward) x (OGL + Vulkan).
But this is just speculation.
Not really because this is now done in XP11.50 with Vulkan! This API makes it way easier to be cross-platform than before. There are still subtleties but these are mostly with Metal, not Vulkan.
Ok, Linux and Mac user will stay will X-Plane. But itās not enough to generate revenue. Anyway, I never understood why user want to use linux for gaming. Donāt get me wrong, Iām a linux user and I love it, but not for gaming. I have a dedicated win10 gaming pc.
Just an example. Thatās pain in the a.. Just for gaming? No one should want games on a linux pc. Security?
They wonāt be ditching OGL any time soon. Try raising that as a suggestion on the XP forums, and see what response you get.
You continue to fly that potao sir if youāre enjoying it.
Dangerous comment. Evidence?
My post above might have missed a point so let me try again:
I believe (donāt know, believe), XP11 vs FS2020 rendering engine discussions are meaningless because FS2020 is more recent and is using more modern techniques without being forced to carrying over legacy rendering engine backward compatibility.
This doesnāt preclude the next X-Plane product, XP12, to have been in development for years in parallel of XP11 updates and to be ditching this legacy burden. XP11 updates in the rendering engine department could have just been a test-bed for XP12 technologies then.
For the rest, many agree X-Plane internal simulation (systems, engines, flight model etcā¦) is still superior to FS2020 and Iād tend to personally agree. XP11 SDK is also vastly superior (for these specific internal simulation components at least) compared to the FS2020 SDK.
[update] Just to be clear, Iām also liking a lot FS2020 for what is good with it and it is certainly growing on me the more I use it. This makes it harder to me to clearly make any choice and Iām certain Iām not the only one feeling awkward and doubtful about which simulator to spend more time with.
Wasnāt there some adage about that, something along the lines of kicking dead whales down a beach. You can do it, but itās not worth the effort. 
Iād say the visible portion of this is what the SDK is offering for system/gauges development, how it is articulated and how the same flaws found in FS9 for some of this is still prevalent in FS2020.
If you need a precise SDK for the development of systems/measuring devices, have a look at German or Swiss SDK (Mechanical Engineering, Aviation). They all copied from there anyway. 
Ok, but why?
I am not a specialist but from my understandings, the release of Vulcan will allow them to start on a new base. Do they really have to reprogramm everything from scratch? For example if they implement a new weather rendering engine, why is it needed to touch the other parts? The big step forward has been Vulcan that will allow them now to continue from there and not to start from scratch. Vulcan was also the probably biggest change in the X-Plane history beetween 2 releases inside the same version.
Again, itās just how i understood it and i may be wrong.
Youād have to ask them. Some forum members were adamant that OGL gave them better framerates than Vulkan, and were horrified at the prospect of going Vulkan/Metal only.
Some plugins, like ālibrainā donāt work under Vulkan either, but I last heard there was a beta release that may have resolved that issue.
Sounds reared. Accept bad fps because of librain?
Itās not the real reason, I was just citing an example plugin that was broken by Vulkan. But they have to maintain backward compatibility for a lot of customers, and OGL seems to be the way to do that, among other things.
In a nutshell. Vulcan is the Graphic API. You need a core engine (mostly in c, c++) and all the tools and SDK. Think about instruments, fligfht characteristics, weather etc.