In my experience, the problem is the investors. Most investors in startups can't stomach the unpredictability, in timing OR payoff of investments in SEO.

In my case, it took 5 years of bootstrapping to hit 1M uniques per month from organic and 7 years to exit for 8 figures. If I'd had investors to contend with, my head would have likely been on a platter well before that.

I believe it. But that's not the same as "it's not a viable strategy".