I've been pretty involved in just about every layer of the stack. I cut my teeth on graphics (and still do graphics) but I've had to deal with networking, input, audio, and gameplay/scripting code as well.
Platform support is likely the most expensive to do right, but I would argue supporting say 2*n engineers per platform is actually cheaper than needing to rewrite huge chunks of Unreal or Unity. I can honestly say with confidence that no AAA game has shipped on Unreal without needing substantial rewrite of various internals. I've consulted on several Unity projects, and it's an actual dumpster fire :/. I've heard that Unity's valuation is driven by cloud service offerings and the ad network they've spun up. It shows in the quality of the code and the investment. That said, frameworks like SDL2 have come a long way in democratizing the platform-dependent bits of code that used to be insanely hard to get right.
I really do believe a better option "should" exist for developers that want to make the game and not the engine, but I sort of lean towards building an engine (leveraging open source frameworks wherever possible) for more "serious" studios that need to maintain their game for several iterations.
It's "Vulkan" btw :) Also grats on shipping!
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