Well before the EC it did just end with Joker and EDI exiting the Normandy and looking to the sky, but people said they were forever stranded and the Normandy crew had to have inbred babies (the Stargazer scene helped their case a bit) whilst the two dextro's, Garrus and Tali, died of starvation. Before the EC everyone automatically took the most negative outlook on the end.
The IT falls apart when you start questioning what would happen after it. People hated such things as Harbinger flying away etc, but with the IT it means Harbinger is still there, Shepard is still injured, TIM is still alive and the Crucible still needs to be used. Plus Indoctrination is not something people have been shown to "break out" of, resist somewhat to say a few lines of dialogue and/or shoot themselves, but not break free of. After the IT a heavily indoctrinated Shepard, so heavy that he couldn't tell the difference from dreams and reality, will have to complete the ending again whilst having Harbinger looming over him ready to kill him. Also, what would the Crucible do? It couldn't just remove their shields, the Reapers could just FTL around indoctrinating planets and employing hit and tun tactics, so it would basically have to be the destroy choice again, but with many retcons of the lore to get you there.
AKA, not better than the current endings.
The ending for me is just the ending, the beings of light theory is nice because it fits within the game, but while I liked the original endings the EC does vastly improve upon it. Would type more/proof read this but need to get to work.
Also d'oh I wrote this on my page 2 days ago.