There are countless instances throughout history of lasting change being sparked by a single moment. Sure, that moment is frequently the culmination of some period of struggle, but you have to remember that the issues that came to a head and sparked those DEI initiatives a few years ago were exactly that—the product of literally centuries of struggle. Or, perhaps more accurately, a recent phase of that struggle.
So, I believe your emphasis is on the wrong side of the equation here. That is, it's not that there is an inherent deficiency in a trending moment or ascendant party giving rise to change. It's the explicit pushback against DEI that is responsible for its unwinding. And, this effort was not successful because the party that sponsored the pushback was ascendant. Instead, part of the party's ascension was due to it making an issue of the pushback. More specifically, the blowback was part of a divisive theme, along with illegal immigration and other issues.
Progress is not a one-way street and gains are not de facto insulated against erosion. Progress (and its security) is a product of the mores and culture of a time, and these can be influenced and manipulated. So, there is really not such a thing as "lasting change", and that's what we saw here. In some ways, the blowback has taken us not just back to our pre-DEI state, but to a pre-1960s mental footing.
You're right that there are tipping points, but they don't come at will, they come when the culture is ready for them. Push too soon, and as you note, you may actually undo progress that had already been truly won.
Culture behaves like a non-Newtonian fluid: manipulate it gently and it flows smoothly. Apply too much stress too fast, and it turns into a solid and resists you. Trump did not invent that resistance, he simply untapped it and rode it to power. The progressive movement created the resistance by applying too much pressure to a culture that wasn't ready.