Ok, so there are a few interlinked issues you've raised here about the relationship between statues, history and the historical record. I'll run through them in the order that makes most sense to me.

First, to deal with the question you raise in text - how important are statues in preserving history itself? As you rightly note, statues (and other physical, public objects) are far from the only way that we can know things about the past. Their primary purpose is a combination of celebration and commemoration, and insofar as they are primary sources, they are sources more useful for examining the attitudes and values of the people who put the statue up rather than the life of the person they represent - quite literally, who they chose to put up on a pedestal. Particularly in terms of the recent-ish past (ie the bulk of statues whose removal is currently a topic of debate), these statues are not important elements of how historians would investigate and write about, say, Robert E. Lee.

That said, these statues would be relevant to a historian interested in the memory of Robert E. Lee - why did so many people in so many places look up to Lee and want to celebrate his life, and what was the cultural and political significance of these representations at the time and ever since? However, we don't really need the physical statues to remain in place to undertake this kind of analysis. Even if they aren't preserved in some alternative way than continued public display, historians can also use sketches, photographs and written descriptions to do their work, insofar as it even rests on the physicality of the statue in the first place. Historians do this all the time for buildings and other physical structures/objects that have been moved or destroyed.

Indeed, if anything the movements to remove statues have greatly enriched the historical record in this sense. One of the things that historians tend to home in on in their work is conflict, not just because it's more exciting than stasis but also because it means that the unspoken becomes spoken, and when things are actually articulated it leaves more of a trace in the historical record for us to analyse. If the statue just exists without anyone talking about it, the historical record will tell us little about how people responded to it or the significance it held for them. But when there is controversy, then a lot more gets revealed. This is of course partly because people have felt the need to defend and justify their attachment to these statues, but it also reflects that resistance and opposition has articulated itself in more forceful and diverse ways. A good example of how the historical record can be enriched rather than diminished is the toppling of the statue of Edward Colston in Bristol, where rather than restore the statue to its former place, it was displayed in a local museum in it's final graffitied, toppled state, along with placards from the protest as well as testimony highlighting the range of responses it provoked (you can read a bit more here). Future historians are going to be able to write about these statues in more comprehensive and frankly interesting ways thanks to these protest movements.

This brings us to the question of revisionism - will these new sources and perspectives act to change the way we understand history? Undoubtedly, yes - that's what new sources and ideas do. History isn't static, and what we know about the past is constantly being revised in some way or another. In a scholarly sense, revisionism is therefore entirely normal - in fact, as a former colleague of mine once put it, what's the point of being a historian if you aren't trying to add to what we already know about the past? What's striking here though is that we aren't really talking about scholarly revisionism at all - if anything, what we're seeing is public narratives about the past starting to catch up to where scholarship has been for years or even decades when it comes to people like Colston or Lee. The current backlash towards the removal of statues is therefore more about politics than history in this sense - whose interests do older entrenched narratives serve? The historical reality of the lives and the ways that they have been commemorated are entirely secondary to their political usefulness, and appeals to the sanctity of the historical record should, in my view at least, be seen quite cynically.