Yeah, I hear you and I’m not against that feature at all myself - in principle -, but your description of what you needed sounded a bit like, well…
Just adding another list for subcategories is a solution that may work for your specific use case, but, for example, in my use case, simply sorting subcategories directly below their parent would be sufficient. Others might prefer an expandable tree. Or even fine-grained control over which categories are shown in the list and which aren’t… 
What I mean is, are you sure that the second list the only possible valid UI design for your requirements? It’s just that the secondary list does not scale well: forums with many top-level categories without children or with very few top-level categories with a huge number of children each will see almost no benefit or even see unneeded, dead UI from this change. A feature request that fulfills your requirements while at the same time degrading nicely for users who don’t need it would have a better chance to be picked as a core Discourse feature, even if it gets initially developed by someone else thanks to your bounty. 