Having only OpenGL/D3D was like if the only way you could do network programming was via python. Python is easy enough to use but it’s not well optimized in every use case. Vulkan is like the C of graphics programming. It’s useful when you want to get in there at a lower level, because the higher level abstractions leave too much performance on the table.
OpenGL was a high level interface, so to speak. And due to this, it made some things easier and other things harder. Most notably, you didn't have the low level access some people/projects wished for. Some time ago, there was a push for a more low level interface and Vulkan was supposed to be the platform-independent, open solution. Naturally, when your interface is lower level, you also need to provide more boilerplate. So Vulkan is slower to get up and running but that's not a particular fault of Vulkan I'd say. The story around it is rather interesting. It is based on AMD Mantle which was discontinued on behalf of Vulkan. Unfortunately, not every vendor was pleased with an open interface. Microsoft released DirectX 12 and Apple released Metal as direct competitors with very similar (if not identical) goals, fragmenting the whole landscape in the process.
Today, Vulkan is supported on Linux and Windows. Apple provides GPU drivers for macOS themselves (not the GPU vendor) and never implemented Vulkan support to not cannibalize its own Metal (I suppose). However, there exists MoltenVK, an open project that creates a translation layer from Vulkan to Metal.
It’s very irritating.
And if they did want to stream a game from time to time (for me it was being able to play on a laptop anywhere, not just on my own LAN), Steam games were usually cheaper from sales and resellers, there was actual multiplayer, and the Steam ecosystem has a bunch of other advantages. It was just a better service.
And GeForce Now had better hardware, ray tracing, customizable graphics settings, vsync, etc.
If you don't need to stream a game, well, you don't need any of these services. But if you did, GeForce Now and Steam were just plain better than Stadia.
GFN is good especially if you live in an area serviced by it's RTX 3080 streaming "rigs" which make it reasonably good value for money vs the cost of building an equivalent PC. But it's still the most expensive service on the market PLUS you also have to BYO library which rules out entire groups of potential users - Nvidia knows this as well based on how hard they're pushing the free-to-play titles. On top of that a lot of titles aren't even playable on it; I have a steam library of ~700 titles and apparently only ~140ish of them can be played on GFN.
Personally I like the GFN idea of being able to "rent" hardware and that's a business model other companies have proven works to a certain degree since ~2015 (Shadow.tech being a good example). However I don't think it has the same mass market appeal of the other surviving services which give you the hardware + a huge library of content just like Netflix/Disney+/Stan etc.