Maybe I should be more specific. I did not want this to be a debate as to what is a fight or what kind of fight you are getting into and why etc. It also is not a aikido does not work in a "real" fight debate.
How about how best to train for a fully resistant opponent.
Questions and comments tend to get over-analyzed and over-philosophized around here, but it's pretty simple. Realistic attacks from someone who is going to resist you as much as possible.
Trade out our traditional attacks for the kind of attacks you're going to see on the street and make sure they follow through. This means you might get hit and it means that if they resist you the entire way, they may fall like someone on the street and that's not going to feel good. As uke, work on the bag a bit throwing those attacks (if they're not natural to you) and change it up. Let uke be completely random in what they do so that you have to react naturally.
Keep in mind, there is middle ground there. You can give a 80-90% speed attack and resist 80-90% of the way and still feel good about what you're doing working for you in a real situation while still maintaining some level of safety for you and uke. It doesn't have to be all out, 100%, for you to know that what what you're doing is effective. Uke simply needs to attack with intent and you need to be decisive and quick in your actions with both of you being prepared for what comes next.
Above all, ease into it, take your time and build up to the level that you want to be at.