Well I wouldn't know how devastating Steven Seagal is in real life
1) Against a common thug:
This is what I am hoping to answer, and I am tending towards yes with more realistic training.
2) Against a trained fighter:
No, but Aikido wasn't intended as such and I don't see it as a failing.
In Aikido we learn to train without resistance but we must train for a common thug's resistance (at higher grades) if we are practicing Aikido also for its self defence element.
In light of the conversation about fighitng, I disagree that our objective is to learn to fight without resistance. In my experiences that has been a failing proposition for me as a strategy. Resistance implies "defense only" without offensive countermeasure being applied as "resistance" would be anything that is offensive. You simply cannot control a fight without seizing the iniative and controlling your opponent, this is offensive in nature and hence you must dominate and control.
Personally what I think gets lost in perspective is that Aikido is a methodology for learning certain aspects of a martial pracitce...it is not a "fight strategy" per se....so when I hear "Aikido is about fighting without resistance". That implies that it is a particular way/strategy for fighitng...and I personally feel that is where we get into trouble and folks start looking at aikido as a flawed methodolgy, when in fact it is not, it is simply being looked at in the wrong way.
Fighting is my life. It is what I do. As such, I have found a place for aikido as a methodology for mastering some very key and important concepts, and frankly, It is challenging slow, but does a good job in doing what it is designed to do which is to teach you how to move your body in very efficient ways.
Integrating this into a "fight strategy"...i.e "cross training" or "MMA", is the correct perspective I think....at least it is the one that works best for me. How do you take your basic fight plan and strategy, ie clinch, kicking, punching, weapons, pushing, shoving, use of mass etc....and make it more efficient?
To me, it requires looking at all the elements and aspects of fighitng and diving in on the spectrum and training each of them under methods of control. Randori is one element, Waza is another element..etc.
I think if we loook at aikido more as a methodology to learn some very important elements of fighitng and less of a method of fighting, it changes how we perceive and judge aikido as a success or failure.