"The reason I think this way is that if I put myself in the shoes of a steem investor, then why would I invest if I can’t use my steem to capitalize off of the blockchain?"

Because actual investors seek to build value into the business so that the underlying investment vehicle gains value, which inures to investors. This is called capital gains, and is disabled by extracting value from the conduct of business. The latter is called profiteering. People that extract that value before it can inure to the underlying investment vehicle are called profiteers. They are not investors.

For millenia, capital gains driven by improving businesses have proven to be nominal means of driving investment. During that time numerous profiteers have destroyed businesses for profit by extracting the value of the business and the value of the underlying investment vehicle, typically stock. While the stock owned by profiteers becomes worthless, their purchase of that investment vehicle has been compensated by the ROI gained via selling the assets of the business, such as the forges, presses, and real property owned by the business.

Steem has no such real property, and the creators of content cannot be sold, leaving only the rewards generated by content available for extraction, which is accomplished via stake weighting such that ~90% of rewards has inured to whales, not reaching the creators of that content. As a result of this profiteering, Steem price is reduced, as is market cap and the user base.

HF21 has increased the financial incentive to extract rewards via stake weighting, and is accelerating the decline of Steem. There are a few thousand active users left on Steem social media presently, and many of those accounts are socks of the original ninjaminers, the whales. As the whales continuously concentrate Steem in their wallets via stake weighting, they recapture that Steem sold while retaining the money received for it from purchasers. As a result, the market for Steem is reduced, and the value of Steem declines.

Due to the necessity of stake to extract rewards via stake weighting, the whales have a dilemma: as the value of Steem declines, the monetary returns from their business model decline in real terms, but they need to hold stake to extract value, and are left holding the bag increasingly worth less over time. At some point, they need to abandon that stake as ROI no longer supports extracting diminishing rewards compared to other profit centers. Until the value of Steem declines below a certain point, they remain financially obligated to holding stake. HF21 accelerating the decline in price enables them to sell their holdings and move on quickly.

I believe this is most of why HF21 has been imposed on Steem: to enable profiteers to move on to other profit centers without suffering the gradual loss of their stake in real terms.