To be fair in practice it's only going to be a relative handful of that 30-40% which are at risk of death or exile. Most regime supporters (especially the rank and file) will either flip the moment they see the writing on the wall or feign coercion as the excuse for their support. Any new government will let them pass in general because there's no realistic way to purge over a third of your population for just supporting the previous regime. Hell the 1979 revolution itself is case in point.
In reality you're only looking at a smaller core of true believers and government/military officials who are the targets - the rest are fair weather and will switch sides once sufficient opportunity presents itself. The challenge as
@Ghostse gets into is we have no way of knowing who all is on the fence. One wrong bomb or one wrong killing and you could take a likely turncoat and drive them back to the regime. Decapitation strikes only work if you have someone capable of immediately filling the leadership vacuum and providing a figure for others to rally behind.