They don't. It's not worth their while. Why offer a firmware update to something that's old, incurring a heap of costs, when they can flog you something newer that will be supported for perhaps two years before they loose interest in it? My Panasonic FreeviewHD PVR, which I purchased in 2013, is still going strong (touch wood!), but was nearly rendered useless a couple of years ago by a transmission change that stopped it recording programmes on certain muxes. Luckily, the change was regressed, otherwise I'd have to not only worked out for a new machine, but sent a perfectly working piece of electronics to landfill for the want of a small firmware update. It's the way of the world.
In my experience, Panasonic is about the worst for supporting older kit, with Samsung a close second. That said, all of the manufacturers are pretty much the same. Think about it: if they produced perfect kit and supported it ad infinitum, why would anyone buy anything new?