Re: Content content content....
Some continental operators, like Belgium's Telenet, have dropped support for older receivers using MPEG2 - and it caused a fair bit of angst among some of their customers. And that's an inevitable side-effect of making HD the default, at least for distribution, which I presume is what you mean.
To drop SD distribution means everyone's going to need an HD receiver; and while 75% of sets may well have HD in the US, that still leaves 25% that don't. Those will be second sets, portables, perhaps things like hotel rooms, and of course the poor and elderly will likely be quite widely represented amongst them as well.
You would potentially be imposing costs on a lot of people who may not be able to afford the move - even if you do provide low cost adaptors; 25% is still a lot in a country so large.
Such a move will come - or if not a move to all-HD, certainly to HD technologies, like H.264 and (outside the US) DVB-T2 - in order to save bandwidth, but it will be slower than you think.
Telenet's pissed off a lot of their customers; I doubt other operators are in a rush to follow suit, and governments won't be keen to tell people - espeically the old ones who vote - that the box they bought for switchover just a few years ago is going to be useless very soon, and they'll have to buy another one.
Operators that supply equipment with their subscription services are best placed to do this; the could upscale all SD material and use HEVC or H.264 to save bandwidth, and many are only dishing out HD-capable kit anyway. I suspect even those will wait a while, gradually swapping out SD-only receivers as they fail, before embarking on anything as radical as a mass replacement scheme.