Well one perspective -- if one could consider a Powercore as a dongle of sorts

then the argument could be made they already have what you're suggesting.
What I found to be goofy about the Powercore solution, at least the last time I looked at it, was that it seemed they had set up the pricing model so that you were limited in how many instances of the Virus VST you could run (regardless of hardware power)... basically they tried to eliminate the cost advantage of softsynths by requiring per-instenace license for the powercore version!! Unbelievable.
With so many good softsynths over the last couple of years I think they are going to have to do something really dramatic fairly soon. Either get total integration working properly in all hosts and get issues like latency resolved, otherwise lunch gets eaten by increasingly powerful CPUs on the PC and increase in synth programming skills by softsynth makers.
I think there will always be a market for a hardware Virus, I just think the potential size of the market is shrinking. They can always continue to make a pricey hardware synth, because its got sort of a legendary-mystical-mythical place in the minds of so many, and I think in many ways it was a very important instrument in modern day electronic music, so just like Moog they can probably always have a high-end hardware product for those who play on stage and such, at a high price point (just less software based producer-types would buy it).