I just don't see any way this change is going to be good. From a business perspective, M rated games just sell better. The gaming market is primarily adults buying games for themselves, so catering to kids doesn't make business sense. Most kids in the teen market now that actively want ME:A are going to be old enough for M games by the time it comes out, and those that won't aren't going to want the watered down kiddy version of it.
From a quality perspective, even if they can manage to not dumb down the story, you are talking about watering down the content and tone and removing avenues for natural dialogue.
You're talking about removing major emotional content with romance scenes by pretending a fade to black has the same emotional impact as a well done love scene, which it doesn't. Assuming romance survives at all.
The only people I see this change appealing to are, frankly, less than a dozen individuals on these forums. And even if they are indicative of a larger demographic, it isn't a significantly large one that I see needs to be catered to on this scale.
If you want to play a T rated sci fi bioware game, there's one out right now. It's The Old Republic, it's free, and it has a new expac coming out in October. Leave Mass Effect alone.