Well, from a game balance perspective, turning the "Harsh Treatment" button into another "Provoke Revolt" button (albeit with a delay) would unnecessarily reduce player flexibility and be bad for the game.
However, I think it's more important to address your conception of what "Harsh Treatment" and "Provoke Revolt" actually are, what these tactics seek to accomplish, and what the end result is in a real-world scenario. EU4 attempts to simulate real-world events in the best way it can given historical context, but I'll be using more modern examples of each to make my point.
First off, you can think of "Harsh Treatment" as a direct, forceful suppression of freedoms. This goes beyond something like "Reduce Autonomy" in which the government essentially makes the sweeping decision extort more resources from the populace en-masse. "Harsh Treatment" on the other hand, is the act of physically controlling vast groups of people, with the intent of disorganizing them, often through brutal/inhumane measures. A real-world, modern day example of this (albeit a dark one) is the CCPs mass suppression of Uyghur Muslims in North-Western China. The Uyghers are kept in restrictive interment camps designed to limit their ability to rebel. They cannot communicate with one another, they are blindfolded to remove their ability to ascertain where they even are, and are moved about frequently, furthering their disorganization. These measures all but remove any hope they have of standing up to their oppressors, and ensures the Chinese government maintains strict control over their actions. The fact that EU4 charges military mana to accomplish this is very appropriate and changing the way this mechanic works would not be in line with the reality of the situation.
In the event of "Provoke Revolt" we can turn to the good 'ol US of A for our modern day example. Specifically, we can look to Portland Oregon, where federal "Secret Police" have been abducting protesters off the street in unmarked vans, often for frivolous or made-up reasons, in an attempt to sow discontent amongst the population. The goal is to stoke the people into violence and give the federal government a "true" reason to crack down on dissent. This tactic has been used all throughout history as a means to eventually justify the use of military force against "unruly" citizens by powerful government officials.
My point being; everything about the way rebels behave in EU4 works as intended, both from a real-world standpoint, and a gameplay one. The change you are proposing would undercut the balance of the game, and would in fact work counter to the way the world works. I hope this explanation was thorough enough.
