Is it worth buying a low end windows phone like Lumia 630 with 512MB RAM? The answer is yes. The 630 does Miracast and has SensorCore which the other low-end Lumias do not have. But the 630 is only 512MB and lacks a dedicated camera button. Check out specifications on the Nokia site.
A dead phone? No, not as long as the OS can run on the hardware. The OS made one change from Windows CE core to Windows NT core. I don’t think you’ll see another situation of users left to burn, like the time when change from WP7 to WP8 happened (that’s when MS changed from CE to NT core), because the NT core is what all(?) Microsoft’s Windows OSs are based on and has been since the times of long gone.
I don’t think there will be any change regarding NT core in the foreseeable future. There will be new hardware coming with new functions, and the OS will be updated accordingly, but it’s my belief it won’t mean your phone will be left to rot. The hardware might be underpowered at some point in time, to take on a new OS, but it’s impossible to tell when that time will come.