My two cents:
I've owned the HD500 since around April of 2011 and a DT50 2x12 since around November 2011. Never really had problems with them, and only using a lot of higher end effects ever bumped me against the DSP limit. The only time I use dual amps is for metal rhythm patches(and then it's just the two amps layered with different cabs to get around the 'meh' IRs) with an OD and noise gate.
Do I have any reason to upgrade? No. Personally I don't.
Is this a good opportunity for new customers? Yeah. I think so.
It'd be cool if Line 6 did a promotional where you could get like $75-$100 off the 500X if you owned a 500(and then I think a lot more people would go for it, because it would cover the cost of the lost value of the HD500) but overall does it really matter? Not really.
If you own an HD500 and like it: cool, it's still an excellent unit. If you don't and want one: an awesome unit just got better, and for the same price as the thing it replaced.
If you hate the HD500: well, there are plenty of other alternatives out there.