I've been link building for a while and focus on content + outreach. I've lost count of all the potential clients and even agencies who I approach that just ask what sites I've got straight away for guest posts. I'm not against guest posting but it gets me down how many people put all their eggs in that basket. One thing, it's so saturated that I think Google are just going to start either penalising sites who offer too many guest posts or use them.
It's also not taking into account that if it's really good content, there's a chance of the content itself gaining links. Why would you want this good content to be on someone else's site?
Building links through content and outreach is not difficult and it's the only real way to protect your site from future algorithm changes.