Last I checked Adobe doesn't have any 3d game engines, so that's not really a solution, heh. (Also, godot is already a FOSS 3d engine which is gradually replacing Unity)
Additionally, developing a game requires many, many different parts. It's not just the engine, it's making the models, it's making the materials, textures, shaders, programming, scripting, writing, and so on. Some, like 3d modeling, already have decent FOSS tools (Blender for example). Others, like the Substance suite (material authoring and texture painting) or Photoshop (image editing) don't have a good enough FOSS alternative to be replaceable. Furthermore, many studios have specialized plugins or companion programs the studios developed or purchased which are only compatible with Substance, Photoshop, etc.
So you're not just fighting game engines, you're fighting every step of the process. Adobe (and Autodesk, fuck Autodesk too) products, when used, tend to be deeply embedded in most professional workflows. You'd have to replace the software and any specialized plugins or programs designed specifically to interface with the software. I mean, good luck, it's just a hell of a lot more complicated than you seem to think.
Fwiw, I've tried to use OpenShot and it sucked hard. Granted, this was on Windows and it was several years ago (3~4yrs), so it may have gotten better (or it might be way better on Linux), but I ended up switching to davinci resolve because OpenShot literally couldn't deal with anything above 720p without crashing. I couldn't complete any university assignments with it. Even just clipping the ends of a video was causing it to crash. Unless OpenShot has gotten significantly better on windows, that's a good example of a place where a pro-quality FOSS alternative doesn't really exist.