The legal principle which holds that the Muslims are obliged to fight against any regime which does not implement Shari’ah has several nuances, and there are obviously conditions regarding its applicability. It is not simple matter at all.
Determining what is meant by “implementation of Shari’ah” is itself a rather intricate discussion, given the fact that we have not experienced complete implementation of the Shari’ah since the abdication of Hassan bin Ali bin Abi Talib from his position as Khalifah.
But let’s leave that discussion.
The basic condition for armed fighting in any set of circumstances, is that there is a reasonable expectation that armed fighting will bring about a better, not a worse situation for the Muslims. If it is reasonable to expect that armed confrontation will worsen the situation, then of course, it is prohibited.
It should be mentioned here that making an objective assessment of likely outcomes does not contradict having trust in Allah, and hoping for His Support.
The theory that the Muslims should plunge into strife and violent rebellion within our societies in order to collapse oppressive State authority so that the rebel groups can fill the void and build a new Islamic system to replace it, is highly dangerous.
The gamble here is whether or not the rebels group will actually be the strongest ones at the end of the disaster. When all of the destruction is local, and losses will all be from the local population (and from the rebels), the only way to be sure that your group will be the strongest contender for supremacy at the end of this process, is if there are no external powers at all interested in dominating the region; because external powers will not have suffered destruction or loss, and will be in the strongest possible position to step into the void your group created.
So no, I don’t think the strategy is reality-based.
Furthermore, as I have written several times, there is almost no indication that rebel groups have any articulated policy ideas upon which to base their new systems. Apart from “Qur’an and Sunnah” and, of course, Hudood, we do not see any detailed plans for how they propose to manage the affairs of the state.
It seems to me that the reasonable expectation, then, is that this approach will indeed worsen, and not improve the situation of the Muslims. It will not achieve an Islamic system, and it will only end the current oppression by replacing it with an oppression of a far more profound and more lasting degree.