I think you're overestimating developer ability/willingness. There's a lot of hardware out there which could in theory have third-party firmware built for it, but which hasn't because it's just too niche for anyone to have bothered, or because there's insufficient public documentation.

Most smart TVs, for example, fall into both categories. There are simply too many models for a third-party firmware effort to take off, and documentation on the hardware used in these TVs is usually nonexistent. (And the hardware is often already stretched to its limits on the stock firmware -- implementing "new functions, including codecs" is likely to be impossible.)