Well, I do think the older parasites were a lot of fun. I also think that those shotgun exploits in CoD sound like they were a lot of fun for the folks using them, though.
The difference, obviously, is that the shotgun exploiters were harming the experience of other gamers, while in a co-op setting that is just not the case with parasites in AIW. But I think that as a strategy game developer we're under a form of contract with players to try and give them the most balanced experience possible. Not because if we don't the game won't be any fun, but because if we don't then the game shrinks. How many people were once only ever starting with parasites, because they couldn't bear to use anything else? Quite a few.
That's trading in a larger body of fun (the whole game in general) for a very much smaller body of fun (parasites only) for a lot of players, which isn't going to make people happy-long term. Once you get bored of the parasite mechanics, what's left? And it's not like we take older versions of the game off the market or something -- the license keys still work on the older versions that have installers floating around out there; we just don't provide support for anything except the latest version.
I'll also note that it's not like we unilaterally decided to change things. We had persistent, ongoing complaints about how game-breaking the older parasites were, and we revised them continually over a span of a year and a half until they arrived where they are now. And, to be honest, I still don't think we're done with them. The complaints now are that they are not as useful as people want them to be, but there's a mixed message in there in that some people want the over-powered fun of them, etc. So we've kind of been biding our time on that, and waiting to collect more feedback from players once they have more time with them.