You don't really need to invest into Steem to use the platform, though. You can read, post and comment with just the free STEEM you get for signing up. Moreover, the main source of potential profit is posting/commenting rewards, and those rewards do not scale with the amount of Power you own, but with the amount owned by your upvoters. As a consequence, there is no profit to be made from investing into STEEM yourself (except for curation rewards, but those are really negligible).

That's why investing into STEEM is not like investing into a startup. The value of a startup increases because of the promise of future rewards, even if it is in the far future. However, with STEEM, there will never ever be any reward for owning STEEM, so the price should not increase.

Therefore, I believe the only reason the value of STEEM is increasing is an unfounded speculative bubble that will collapse sooner or later. Of course, large Power holders will say otherwise, because they need to keep the value of STEEM up for as long as possible to cash out, because powering down can only be done at a 1%/week rate.