Imageboards are effectively dead outside of 4chan. They either have no activity, or the activity is so bad you don't want any part of it. They're run by either unpredictable schizos or imbeciles (/v/, /a/), or people who don't want to moderate at all (/tv/, /g/). They are dominated by nutjobs and retards with infinite free time and energy. They are unstable, and people are tired of the outages, shutdowns, migrations, splits, merges and bunkers. What few users are left keep fragmenting more and more. 4chan isn't much better but it's stable and active, and good enough for most people (not for me though, I'm permabanned).
I don't think it's possible anymore to build an alternative imageboard of any significance. There could have been some kind of TechChan or gChan made years ago, but instead we got lainchan where you can talk about the wonders of dilation, antidepressants and communism, and other low life & high tech topics for hackermen.
If people hadn't gotten so used to having constant social interaction and other stimulus on the internet, most of those still humping the putrid corpses of these shitty imageboards would have left by now. They're only here because they have nowhere else to go, but still need to be somewhere.
Someone once talked about people who "live in their browsers." That's something I've been trying to get away from, and I shouldn't keep checking Cuckchan and Fuckchan and Garbagechan just for the satisfaction of seeing that someone made a new post that I don't even want to read. We should all just leave.