I guess it really depends if Line 6 has managed to already tap out the processing power of the HDs. In some ways they have, of course (it's not like you can chain together 5 spring reverb units without maxing the DSP), but for the most part the HD500 has been able to handle a lot of what they've thrown at it, including the deep editing parameters, Dream Rig capabilities, etc.  If they can get away with software upgrades to sell more units, it's a hell of a lot more cost effective for them then designing and manufacturing a new unit.