Onsite SEO is so important. About 80% of your success will come from onsite SEO. I have proved this many times to my clients. The problem is there is a lot of bad information on the Internet when it comes to SEO. Plus, there are many companies that just sell a backlink service because it is very easy to sell and an easy service to provide. All they are doing is running automated software to build crappy backlinks that will eventually hurt your website.
Backlinks do help when they are naturally built, but there is no way only backlinks will help a website dominate Google. Logically it doesn’t make sense because of Google algorithm.
I can prove how important onsite SEO is because there are no websites that get high Google rankings strictly on building a lot of backlinks with no onsite SEO. But I can show you plenty of sites that have high Google rankings with great onsite SEO and no unnatural backlinks.