Do you think the Crown (in any era) would consider partitioning a region between loyal houses after the rebellion of a Great House in order to gradually diminish the Great House system and their hold on the 8 regions? For example, Stannis could split the Westerlands into different subregions instead of just naming another house in control of the Westerlands he could split it into five subregions or something

Maybe, but I doubt it. It’s possible that partitioning would allow for divide and rule, but that will lead to a lot of direct governance from King’s Landing, and I’m not sure that the Iron Throne has the capacity to do that.

So the danger is that in trying to keep the Westerlands down, you’re creating only a “hollow strength” that will vanish once one house eventually reunites the others. Because the problem is that there aren’t much in the way of natural borders inside the Westerlands, so you’re going to get a lot of fighting over territory, and chances are someone will win.