As we know link exchange has been closed by mostly sites ..so in this case how can you assured that link building is just 50% of SEO activity...there is lots of techniques to promote your sites unlike link building only...
in seo on-page and off-page optimization, then link building is one of the part of off-page optimization.
SEO can get pretty complicated, but link building certainly does play a big role in it. It's something that takes a lot of time too, you need to always be working on it, this will give you the best rankings since it will look like dynamic growth.