I dunno if I agree it's doomcoding.

I have worked briefly in the game industry long ago (not in engine teams though) and ended up writing a few engines for myself after leaving, a couple quite complete.

It’s just a hobby like another. I have zero interest in writing a full game on top of it, or even publishing it.

But do you know who NEVER gives me grief about it? People I know who work at Unity or [previous employer] writing AAA engines.

My last monstrosity is a Rust one with zero dependencies other than the std-lib/OS, Metal/DX12/Vulkan, its own physics engine, spatial audio, vm for scripting and other stuff. I will probably do nothing with it other than having updated my skills in a lot of areas.