You are of course absolutely correct that Rudy's contribution has been minimal at best. Then again, we are seldom playing to his strengths. We didn't utilize him properly last season, nor have we thus far this season. I honestly think he gives us a much needed, different tactical alternative, considering the rest of our 'strike force'. And that is the sole reason I'd still keep him around, because with the right kind of ammunition, he'd surely score plenty. There's no denying he is as good as they come in the air, even though his general movement mostly is quite poor.
Ayew on the other hand, really is surplus to requirements. We have plenty of players who offer exactly the same, and more.
Ps. £6M move to Boro more or less confirmed. That's that then. Good luck finding a replacement for that figure.