Those reasons you have already stated is why Cena/Edge just doesn't sound that appealing to me. Plus, they just had another match with the two just last week. They killed any chance of that working now by just having a standalone match. The story here is far from as compelling as Orton/HHH is, that's all I know.
Honestly, if they would have just held their guns on waiting with an Edge/Cena match at Mania without last week's Raw to muck it all up, I don't really see what would have been so wrong with that in the first place. I'm not really feeling it, but considering they always have good "big" matches, I could accept it. But inserting Show just kills it.
Anyway you slice it, this is just totally underwhelming for one of the world title matches at the 25th Mania. HHH/Orton, I feel like they have at least worked hard on the storyline behind it to justify the match, but this is the complete opposite.
As far as Cena/Edge/Show goes........same reason as was also stated as well, it's not Mania scale, anyway you slice it. Edge/Show or hell, even Christian/Edge or something ridiculous like that and Cena/Hogan or Austin sounds much better. I just can't see how putting Show in another Mania main event is warranted at this point, more than anything. If Batista would just be able to go by April, the obvious choice for Cena would be right there.
Edge/Christian could have been GREAT IMO.. too bad it was sacrificed for the sake of swerving the internet smark fans ala WCWs miserable russo days and we got Matt Hardy on smackdown instead of Christian