Well, the main issue we had with virii farms was it generated a huge amount of HPD while unlimited virii could be installed.
Well since we now have remote malware, a consist malware bombardment would cripple any virii farm. If you make the virii target mega-corporations, then you could make missions like the Kill the Baron rewarding players for disinfecting private virii farms. Along PvP missions and the IP dealer no server can stay hidden forever. So I do not see any real danger in just making it unlimited.
So if we have unlimited virii farms, at some point the virii income becomes greater then mission income. That's where it gets too easy and you lose interest.
Well I do not lose interest because I still need to do missions for AP. I do not lose interest because I have to still keep up with everyone's software while researching better software. When was the last time we had over v1000 software?
I mean why can't we have nice things?