I don't think Eric was an awful idea, what was awful is adding Eric as a surprise, people at first expected another normal incest game but the creator didn't want it to be so simple. I think that if from the start the creator said "I'm gonna add a rival" everything would have been better.
What I understand is that people expected something from the creator and if he adds something that those people hated... well I don't need to say more about what happens do I?.
Most games are quite so-so because the mother always remains single during 10 or 20 years and I'm like really?. The only thing with Eric is how incomplete he is and how much of an idiot he became so that you could kick him easily out of the house.
But meh I just wanted to say that Eric in itself isn't an awful idea, you may like it or dislike it but I'm sure it's based on yourself rather than the idea. The thing in adult games is that most people just want an easy way to the sex scenes without winning against anything, no challenge at all.