Back-paths and shortcuts are all well and good, but it all depends on how the plot is written. You kind of need Martin Septim to stay alive to solve the Oblivion Crisis. I suppose they could have added a back-path to Skyrim's main quest if you could still learn Dragonrend and get to Alduin's "hiding place". And then you've got situations like the College of Winterhold where the entire plotline is completely screwed if you kill the Ancano the first time he condescends to you. Honestly, I don't like the idea of taking shortcuts just for the sake of taking shortcuts. They added a back-path to Morrowind's main quest so that you could still beat it even if you killed an "essential" character or otherwise completely screwed yourself, not so you could just skip an entire questline.
I am hoping they don't just sort of plunge us into the main quest so readily. I never really felt like there was an appropriate moment to go out side-questing with the sense of urgency the Oblivion Crisis had. Skyrim did it a little better, although there was so much non-main quest stuff you were completely missing out on if you just ditched Hadvar/Ralof at Helgen, like the Thu'um and the Civil War questline. There should still be a threat to all existence, sure, but let us rise to the call instead of throwing it on us. You know, we escape prison (I'm willing to bet a Thalmor prison) and instead of "we need you to do this thing", it's "your character has motivations to do this thing of their own volition, but hey, you just escaped prison, live it up if you'd rather".
Not sure how I feel about non-linear questlines. Daggerfall's questline was great in that everything sort of started itself naturally and it introduced you to all of the major players in the Iliac Bay, but there was very little autonomy to it. Redguard probably did it the best; you start out just following different leads, and all of those threads eventually come together. Non-linear isn't a strictly old-school mechanic, and it doesn't have to interfere with the grand narrative the series depends upon (things have to end a certain way, unless we want to see another Deus Ex Machina like in Daggerfall).