HD Subcarriers (HD-2, 3, 4)
You need to have content which the public wants and will seek it out. Those stations which have employed translators to get their HD-2 formats into the the analog realm have found them to be many times viable. And some have used the HD-2 to broker out the time to those translator owners. Iheart and EMF has a couple deals here where each supplies programming to the other's translators. But again, it's the content which matters.
Some content is so niche that there is no sustaining commercial mechanism and they just fade away. Never mind there are some significant technical and business matters which need to be addressed by the host station. Is it worth the effort when the DTS (or who ever owns HD's patents today) takes 3% off the top for license fees of all sub-channel revenue. For stations which simulcast other co-owned stations to extend their reach, there is a lot of legal banter as to whether the 3% applies to the total station revenue of the relayed station. And that's before all other music licensing fees now totaling something more than 7% between all rights holders agencies (both performance and composers).
So station only net's 90% of every dollar earned before all their expenses. In many instances, that 10%, or even 3% margin makes or breaks the operation. More often, it breaks the operation. Especially in more marginal markets.
So...the reality is, as someone else put it, HD is a lot like AM stereo. Lots of potential, but a lot of fizzle in the end. I can go on and on...but...