most self defense situations are often time initiated by people looking to prey on the weak and seemingly unprepared. most of the time the assailant expects very little if any resistance from the victim. the effect of the element of surprise in this context is even more potent ime. of course this isnt ALWAYS the case ie. Deranged/mentally ill and/or drugged up individuals might be determined enough or crazy enough to not care if someone resists and continue their attack.
more important than any weapon is yes knowing how to use it but also having the situational awareness to be able to be ready to either 1. remove yourself from the situation if possible beforehand ir 2. be prepared to engage with determination and hopefully some basic level of skill to take care of the threat.
whether its a gun or knife if you lack the situational awareness to even see whats coming to be able to deploy the weapon and effectively take care of the threat it wont matter what weapon you have.
as for knives not being a good self defense tool. i tend to disagree. Anyone can stab and slash someone even without training and that tends to make even the most aggressive think twice when they are stabbed in the eye, throat or groin. Knives do not malfunction or run out of ammo, and they are easily concealed and quick to deploy and more importantly you can carry them pretty much everywhere.
Weapons you dont have on your person are pointless, a lot of people carry knives in day to day life even for everyday tasks. Also a good majority of people dont have concealed carry permits or even might live in a place where a firearm possession is totally out of the question.
Weapons are useful in their specialized roles and the appropriate context. for instance if im in extreme close range, even if i have a firearm a knife would be quicker to deploy and more effective than trying to fumble around with my gun which is concealed under my shirt, rack a round, take aim, fire and hopefully not get tackled or the gun taken away in the process.
As far as "knife fights" go... this is coming from someone whos trained in offensive and defensive knife tactics extensively, if i could help it in any situation ESPECIALLY when dealing with a knife or any weapon for that matter, my goal is to not sit there and have an exchange with the other person. Strike hard, strike fast and get the fuck out if possible. in most cases unless you are a soldier, cop, security professional or a dense knucklehead who likes to get himself in to shit i doubt average people facing a mugging or sexual assault situation are going to be in armed combat 1v1 because of the reasons i stated earlier on.