I think you're viewing books and online content as entirely equivalent media forms; which would be much the same as treating movies and theater as equivalent content - they're adjacent, and provide similar material, but don't meet the same ends and their method of consumption are wildly different.
True, long form reading is also a skill that can be built up, and that skill even varies from topic to topic. Anybody that's done any academic reading (a.k.a. reading through dense academic journals and papers) will tell you it's a learned skill that is vastly different from either non-fiction or fiction reading. I can easily pound my way through a 1000 page book from Neal Stephenson in a few nights, but I need weeks to dig into a single paper in Category Theory, Non-linear Systems, or Control Theory; and it's because reading those academically isn't a skill I've built up yet.
And while it's true that much of the synopsis information on a topic can be found online, and you can get the details on Grapes of Wrath from piles of YouTube'ers and CliffsNotes; there is detail and meaning that you may take away from that book that you wouldn't find in those short reviews (they're great for school reports in a pinch, but not great for actual contextualization). So, in all, it also depends on what you're reading for.
And that brings me to my last and probably most important point, "What you're reading for." I read something like Stephen King for enjoyment, plain and simple, there isn't some meaning of life that will probably be gleaned from it. I have a friend that reads piles of "bodice rippers" for enjoyment, they read a lot of other things, but that's their junk food reading. I read various pop-science or pop-psychology or pop-history type books for a mix of enjoyment and seeing if I want to dig deeper on the subjects they're covering; and they can be better (and sometimes way worse) at conveying interesting points on a topic than a quick few articles online. I read academic journals to further my education and keep up with new things coming out in my fields of interest. And like most of us, of course I plow through online content, news, the Twittersphere, and bloggers I follow for many of these same reasons.
So it's honestly up to, and on you, for the reason you read books (or don't). Reading of any kind is a learned skill, and can atrophy in the same way your muscles will when you stop going to the gym - so if you want to read books, go for it; if you don't don't; but just like going to the gym, don't let the first few weeks/months/years/decades/lifetimes of soreness stop you from the goal you're interested in.