"and as that is the most likely outcome, why then do we need defence forces at all?"
Because, in the future, as the balance between the USA & China gets closer and closer, the Yanks will only come to the defence of close allies. If we aren't in that position, we don't get their support / enjoy their security umbrella. If we don't get their support, it opens the door to be coerced.
That's the whole point, we need to be close enough that we can rely on their support/deterrence.
Think of it like this. Russia wouldn't attack a NATO country, but it's happy to attack Chechnya, Georgia, Ukraine etc. Nuclear powers won't come to blows, but that doesn't mean a nuclear power won't coerce / attack a smaller non-nuclear power for whatever reason.
