Rebels eventually go away if you ignore them and they can't enforce demands lol. That is really annoying though, much more so during westernization when you've colonized islands throughout the Indian ocean and into Indonesia and are say Sub-Saharan. At that point yes, a 10 stack on every colony *is* unreasonable. Your entire force limit would be used up before you can cover half of them, and those rebels are very strong compared to your forces. Fortunately, current westernization allows you to set it up so your only rebels are events, so that's a lot more doable these days.
I abhor mid-war AI rebels popping out of occupied territory and helping it far worse because of how effing stupid and senseless it is that a nation's hostile rebels would make an active effort to protect it (IE patriots of a nearby nation, peasants angry with current rulership, etc). Yet another Euro-centric trash requirement is that you're at tech 3 to support these guys away. India, China, and Africa are powerless against such rebels and are rarely large enough to simply destroy them outright, barring regional powers like Ming, Vijay, or Mali. But a Euro OPM or even a horde? No problems there; just pay them .1 ducat/month and they are friendly! Derp!
As if rebel support as a concept isn't the territory of pre-ADs concepts. Why is there even a tech requirement on that?