But Monte Cook's blog is not about whether or not it makes for good design to have systematic ways of injecting complications; or whether or not the players should sometimes not succeed. It is a criticism of one, particular, mechanic: the critical fumble in which a bad roll always results in the character performing incompetently.
To precise, Monte Cook's blog is about his own system, which is a critical fumble system and is described as such. It is not a fail forward system, as his own discussion makes clear:
"In a combat situation, a GM intrusion can range from the opposing creature gaining an additional chance to attack for a round, to reinforcements for the opposition showing up. It could mean that the character accidentally shoots a friend, or drops her weapon, or slips and falls, but those should be rare. Far more often, it should be some external circumstance that arises, and not something “wrong” that the character did."
The open ended nature of a the GM intrusion system means that you could use it as a fail forward system simply because any time you insert GM fiat, you can do anything, but it would not in my opinion be a very good one. (Which opens up the question not just of what a good fumble system looks like, but also what a good fail forward mechanic looks like.) At the very least though, even if you start using the GM intrusion system to handle fail forward, it would still be in addition a fumble system, as again his own discussion makes clear.
Here's Jonathan Tweet expressing a broadly similar sentiment in his preface to the 20th anniversary edition of Over the Edge:
A simple but powerful improvement you can make to your game is to redefine failure as "things go wrong" instead of "the PC isn't up to the task." Ron Edwards, Luke Crane and other indie RPG designers have championed this idea, and they're exactly right. You can call it "fail forward" or "no whiffing."
To be blunt, I'm not sure Ron Edwards has been exactly right about anything. But in any event, it is not an improvement to a game to redefine failure as "things go wrong" instead of "the PC isn't up to the task" if we are to apply that idea universally or to all games*, nor is it in practice anything but a semantic difference in most cases. Things would not have gone wrong if the PC was up to the task is generally a fact of such systems. Moreover, I consider there to be a very big difference between "fail forward" mechanics and "no whiffing" mechanics. I likewise suggest that "fail forward" is best implemented as a scene based system to handle what you might call "scene failure" and not as a task based mechanic for handling "task failure". And in any event, even if you can have a "fail forward" mechanic on task resolution, it's abundantly and undeniably clear that the mechanic in question is not a "fail forward" mechanic but a "fumble" mechanic because GM Insertion DOES NOT occur on mere simple failure, but extra complications are inserted in the event of catastrophic failures. Likewise, the insertion of complications does not imply the story must go on in this system, however much you may want to kludge this into your pet theory. The facts don't fit your case.
You can assert that there is no significant difference between the RM/MERP-style fumble rules and the sort of system that Cook and Tweet (and their predecessors Edwards, Crane, et al) are advocating. But I don't think that would be the experience of many of those who have played both sorts of system.
I assert that there is a significant difference between RM/MERP style fumble rules and a proper fail forward system, but that the system in question is not a proper fail forward system because it is not meant to address the problems that a fail forward system addresses. Again, if it did, why does it address tasks and not scenes, why is not geared to advancing the story, and why does it only advance the story in the event of catastrophic failure rather than simple failure of the task? "Opposing creature gains an additional chance to attack for the round" is not a fail forward style consequence, so stop acting like it is. You can hammer your square pegs into narrativist jargony round holes all you want according to your usual pattern, but no matter how much you try the mechanic in question is not the same as the one Jonathan Tweet is talking about in the 20th anniversary edition of Over the Edge, nor is it "no whiff", nor is it "fail forward".
All Monte Cook said was simply "catastrophic failure doesn't always have to make the character look inept". Even if I did agree with that, it's mainly his reasoning for getting there that undermines the argument. But in point of fact, catastrophic failure unavoidably makes a character look inept so the whole point is silly. And that's even before I get into the uses and misuses of "Fail Forward", which may well be informing Monte's thinking here, but which is utterly inapplicable to the actual rules system he's talking about.
*Let me just go ahead and prove that statement since I know it will get your britches in a wad:
a) If your task resolution system doesn't allow Han Solo to whiff, then your task resolution system doesn't allow you to recreate the fiction of Star Wars
b) If you can't recreate the fiction of Star Wars with the system, then its not well suited to being a system for a Star Wars inspired RPG.