Remember that powering down takes 2 years with Steem -- that means your investment will be fairly illiquid for quite some time.
Steem seems poised to grow significantly -- which is a good start. But there are other factors that will ultimately determine price: what is the inflation rate? how many people are powering down? is Steem able to stay hack-free?
Diversification is good -- it's why I've invested in Bitcoin, Ethereum, and now Steem. All of these are speculative investments, but it seems unlikely that all three will fail.