All of them will get it.
They are changing the minimum spec requirements for OEM's for phones that will come out with Mango installed, as they will from time to keep the hardware of the platform modern & moving (like new gyroscope support, more recent chip technology). Otherwise OEM's would just use 2010's minimum specs until stocks runs out.
Eventually the older hardware will not support newer builds (like original iPhone vs newer models), but thats just how tech works. Eventually the processors in todays phones won't be able to handle tasks on future phones; or have hardware components necessary to do so.