Well, yes and no. Sure, you can get hit with an algorithm change or penalty that destroys all your traffic. However, if you have good people who know what they are doing, this is not likely to happen, and if it does, it is easy (in most cases) to get your visits back. Panda and Penguin are another story, but if you get hit by those it is typically not accidental.
Of course, you can buy traffic but if you won't build up organic traffic sources over time, your business will become dependent on the amount of budget you spend on advertising. And advertising costs doesn't stay the same. Most of the time if it gets more advertisers, the costs are going up. And suddenly your profit can disappear and ROI can become negative.
Google is still the most popular website on the internet. Google is mostly recognized as a search engine and their main goal is to provide the best quality web search experience for its users. That's why Google algorithm has a list of requirements and rules for ranking search results. They want to provide the most accurate and quality answers to every specific requests by its users. You already understand that competition on the internet is huge. SEO is still one of the top ways of driving natural traffic to websites but it became hard as hell to get your page to the the leading positions in search results for your focus keywords. And the harsh truth - 90% of all the attention goes to first two pages of results, and 70% of that 90% stays only on first page. So SEO is a long term strategy and you should focus on creating valuable content for your visitors because it is the key factor for SEO and content marketing is the base of most of other organic traffic strategies.
Secure (https) to non-secure sites (http): Since Google began emphasizing the importance of having a secure site, more websites are securely hosted, as indicated by the “https” in their URLs. Per the security protocol, however, any traffic going from a secure site to a non-secure site will not pass referral information. For this issue, you can correct by updating your site to be secure through a third-party SSL certificate.
Keyword Relevance – It’s critical to choose keywords to optimize for based on how relevant they are to your products or services. If your keywords are not intrinsically relevant to what you're offering on your website, the traffic the search engines feed your website will not be delivered a relevant message. Thus, they will be unable to complete the “search and reward cycle.” You see, searchers seek relevance and without it you're unlikely to convert them into customers.
Additionally, knowing how your SEO is performing can help you make the necessary changes over time to keep your rankings high. Look around for a keyword tracking tool to help you track how your SEO is performing. It’s typical for your ranking to fluctuate from week to week and even day to day. Look for a general upward trend from month to month that shows that your efforts are successful.
Google claims their users click (organic) search results more often than ads, essentially rebutting the research cited above. A 2012 Google study found that 81% of ad impressions and 66% of ad clicks happen when there is no associated organic search result on the first page. Research has shown that searchers may have a bias against ads, unless the ads are relevant to the searcher's need or intent 
Organic traffic is the primary channel that inbound marketing strives to increase. This traffic is defined as visitors coming from a search engine, such as Google or Bing. This does not include paid search ads, but that doesn’t mean that organic traffic isn’t impacted by paid search or display advertising, either positively or negatively. In general, people trust search engines, and sayings such as “just Google it” reinforce that humans are tied to the search engine. Thus, paid search, display, or even offline campaigns can drive searches, which may increase organic traffic while those campaigns are running.
Backlinks are basically Authoritative linking. Which means someone else says about your site that it is in an indication of a particular keyword or you have authority in a particular market is indicating that their readers can go and find more helpful information from certain places on the web and they do that by creating these authoritative links which also called backlinks. The more of high quality, authoritative links that you have, Google considers this as you are being incredible in the market. Your website can be authoritative by having other website owners to link to your website, Then Search Engine algorithm will consider your site and you will get higher boost to your SEO and your site will likely get higher ranking and the more of this authoritative link. Blog Commenting is a great way to get backlinks to your website. Step 1. Find relevant and high traffic blog in your niche. Step 2. Actually read the post, what all it’s about. Step 3. Just leave relevant comment to the topic, then simply place your link in the comment.
Plan your link structure. Start with the main navigation and decide how to best connect pages both physically (URL structure) and virtually (internal links) to clearly establish your content themes. Try to include at least 3-5 quality subpages under each core silo landing page. Link internally between the subpages. Link each subpage back up to the main silo landing page.