Modern operating systems are built on the principle of abstraction layers. The *nix kernels place devices as memory pointers on the filesystem tree (they are not files however in the literal sense), so you can swap out hardware and kernel but maintaining the same pointer names and your upper level application won't care at all, as a matter of fact it may not even know it runs on different hardware from before. The "ROM" that you download and flash onto your device is not monolithic, it's made up of components and the Linux kernel and Android OS, and the rest of the applications are separate parts. You should be able swap out the hardware specific layer, otherwise the design of the OS won't be worth a damn.
From user's perspective however "ROM" means something else, it means a specific set of features, be it pre-installed apps, eyecandy (like Sense or Touchwiz), or other miscellaneous bloatware. Basically it's a snapshot of an environment, which defines the user experience. HTC has their own tailored user experience which they ship with their phones like the One mini, with Sense, and probably some carrier restrictions (like custom restricted Wi-Fi tethering on AT&T). HTC first, because it's a product manufactured FOR Facebook, and marketed as A Facebook phone, has a different user experience from typical HTC products. Samsung has their own tailored user experience with their own apps and Touchwiz. Finally LG makes their own phones too, however the Nexus device they make for Google is a Google product, and Google tailors the user experience for their product. I like Google's vision of the smartphone user experience (I had a Nexus One before), and that's what I want to achieve with the current phone. True, that I could have bought a Nexus 4, however HTC's hardware design allows me to do things that are seemingly impossible with LG... Like swapping the IMEI (don't ask).
Not everybody wants their user experience be of HTC or CM flavor. Please be respectful of that. Arguing against wishes of others accomplishes nothing.
I believe it's clear enough that One mini updates can AND WILL be ported to our phone, there is no point to discuss that further in the context of THIS thread.