Check in the before and after history of the Middle East for the following
Iran Revolution
Iraq
The Arabic Spring Countries
First off, each country has it's own circumstances.
Second, you romanticise how those places were before.
Third, if you're gonna mention 1979, you can't ignore Juhayman's seizure of Mecca. That arguably had as big an impact as the Iranian revolution.
The lesson to take isn't that these places were better off with the status quo, they were ripe for change, the lesson is that regimes either proactively respond and change or risk things getting ugly.
Saudi Arabia without the monarchy would be like shangri-la for ISIS.
1) Kill king and eliminate monarchy
2) Seize power in the vacuum created
3) Control holiest site for muslims and have the easiest to recruit demographic
bada-bing
bada-boom
Nah. ISIS are a fringe in Salafi circles. The mainstream salafi's won't risk all the power they've consolidated over the years.
The biggest risk if there's sudden regime change is the breakup of the country.
If we're playing what ifs...
The Hijaz, home to Mecca and Medina, will certainly try to secede, as would the oil rich east. In an attempt to keep hold of them, Najd will play the sectarian card against the Shia in the east, and the tribalism card against the cosmopolitan liberal elite of the west.
Neighbouring countries will get sucked in. Jordan and Egypt will use historical and ancestral claims for the Hijaz.
Iraq, Bahrain, Abudhabi, and the US will intervene in the eastern province.
We haven't even talked about the mess that's the south.