Well you see, in that sentence there is the heart of pretty much all the problems in CiV diplomacy: The AI doesn't value long term friendship. You can form Declarations of Friendship and Denounce all the same leaders, when suddenly the AI will for whatever petty reason Denounce someone you made DoF with or befriend someone you Denounced or someone who Denounced you, which will make the whole alliance thing crumble because "you denounced their friend" or "someone they like better than you denounced you" or "you made a DoF with someone they denounced" modifiers will start popping up all over the place.
But while it lasts its very workable.
I don't understand the fetish with permanent long-term alliances. I suppose someone can make a mod with permanent alliances.
But if you play the normal game, the reason alliances aren't locked forever is because they can shift over time as interest change, and more often than not I know exactly why they change. There's various flashpoints over diplomacy. AI competes/coups your CS, AI invades any AI you are friends with, you invade someone making the AI feel uncomfortable, you click yes to every friend request, pissing off everyone and making you look like a cheap prostitute. A lot of it is two sided. Obviously the human side is dependent on how you react to AI invading buffers empires, killing/crippling your trading partners, stealing your CS etc. but the biggest change in Civ5 is the AI pretty much is programmed to react negatively to all of the above as well, rather than standing there and grinning with a -10 we hate you modifier in the diplo screen that means nothing.
The only reason people seem to be upset is that in the old game, they're like the popular jock who did the dumping (of allies); so Civ5 throws a wrench in that and your long time ally may decide to part ways mid-game. Rage!
Will the rank of the spy be important ?
I mean will a high ranked spy have more change of succeeding a coup.
And will a high defensive spy have a lot of change to counter it?
AFAIK, only steal techs and defending against steals levels up your spy. CS shenanigans do not, this includes rigging elections.
Now the proposed patched will allow us to defend against coups. But nothing there says it will always be successful, only that you get about 2x more success rate couping a CS without a defending spy.
So a number of possibilities here. They upped the success rate of couping an undefended CS, they reduced the success rate of couping a CS with a defending spy, or they did a bit of both. Either way this is a much welcome change.
Also note the reduced coup frequency in the patch notes preview.