There are a lot of people who are older and whose voices change or become limited in range, but most of them will adapt their style to fit that range or change of timbre or whatever.
Roger is suffering from that as well and has been for a few years now, hence the whole lip syncing thing, but it's just another way to ensure that you can sing an old song without sounding like crap.
The first idea that struck me when I watched this clip was that if Gilmour would have sang with him, it would have sounded a lot better, not only because their voices sound good in harmony, but also to keep Rick steady because it's far easier to sing along with someone than for one to carry a tune by themselves, even for some of the greatest singers in the world.
Also, David's voice sounds more like Syd's than Rick's does, so it would have provided a more identifiable sound and that's very valid because most Pink Floyd fans are after nostalgia since the prospect of a future for the group is bleak. Any way in which they can make their performances sound close to the original without being too close that it's boring will provide for a much more pleasurable experience for most fans.
It's all we have left besides ROIOs, reissues and occasional one-off performances with one or two members such as this.
