For a historical perspective, most of our current mantra of "diversify" stems from the days before low-cost index funds and when a person's entire portfolio might consist of 6-20 individually picked stocks. Until index funds and discount brokerages came on the scene, it was cost-prohibitive for most individual investors to own more than this.
Is VTSAX diversified enough? Depends on what you are going for. As noted, it's a market-cap weighted index, which means the top ten companies will be worth more than the bottom 2,000+. But here's the rub... the very reason why Facebook or Amazon holds such a massive position relative to, say, HR Block is because it is WAAAAY more important to the overall economy. Amazon has a quarter of a trillion in sales and over a million workers. HR Block does does about $4B in business and hires 90k mostly seasonal workers.
If your goal is for your portfolio to reflect the broader economy, which holding should be worth more?
Or to repeat what
@ChpBstrd said... what reason(s) do you have for changing your holdings to reflect something not representive of the economy? You can absolutely to increase holdings of some sector or of smaller companies, but you should be clear about the reasons why.
Holding a bond fund, for example, has some very obvious, well researched reasons. Deciding you want more small cap companies at the expense of very large companies which employ millions (e.g. Apple, Amazon, Microsoft)... well ... why?