Why doesn't canon make a cheaper 500mm that is only 5.6 and without IS? It would make it far more accessible. Like the 400 5.6
First of all, did you ask someone at Canon...assuming you can find someone there involved in that kind of decision making? Because unless someone at Canon reads this Beginner's forum (unlikely) then I think you're out of luck for a real answer.
However, so as not to make this a complete waste of your time, why not get the 400mm f5,6 and a 1.4x teleconverter? That'll give you two useful options: a 400/5.6 and a 560mm. You'll lose an f-stop or two because of the teleconverter, but if you're shooting digital you can try using a higher ISO to buy it back.
I'll leave it to you to do the math on the prices.
Just a thought,