Beyond the debate on whether or not downvotes are useful as a tool, I think about the higher-level implications of a change like this.
One of the most powerful media platforms in the entire world can institute a change like this under everyone's watch, and none of us have any recourse.
A couple of programmers in their two-week sprint can fiddle around with some Javascript and push it out to the masses, and suddenly large swathes of the historical record are gone forever. (If you consider downvotes to be historical, that is.)
Imagine if someone could white out a section of a book and apply the changes instantaneously to every printed copy of that book in existence.
I think about the implications of being able to look back on human history, and how that historical narrative is increasingly in the control of technology companies and a relatively small amount of people who decide to change that narrative for everyone else. I remember many chronological videos that specifically call out the dislike ratios of some YouTube videos as evidence of the negative backlash certain companies or individuals have received. With no dislikes, that point can no longer be made. That information is lost.
During a live recording of a Japanese entertainment group I watched, whose videos will be preserved on physical discs and be watched by thousands for decades to come, they announced on camera that, in the span of their two-hour performance, they had reached trending status on Twitter. All they had to mention was the word "trend," and the audience responded with enthusiasm. The influence of Dorsey's creation has permeated so many of our lives, including the people who, ten years ago, would never have cared about what bizzare-sounding technologies like Ruby on Rails would eventually enable them to obtain: a universally understood, instantaneous signal of approval.
I believe that the record of who we were and what we did will be important for us to be able to look back on, as it has been for centuries. But it seems that, in the coming decades, that will not always be up to us to decide - just a couple of people in the engagement bureau who decided for us that the alternative was better.