My perspective and opinion...
The Martial Art is doing the right thing at the right time. We do not have to know every move, or be an expert at them all. We just need to know enough to respond successfully to any potential attack. If your attacker could possibly attack you with one of 10,000 different options, you don't need to have mastered 10,000 different responses. You could very well have perfected a dozen or so, which deal with each of a few general types of attacks as have been mentioned (striking - long, medium, or close range; holding - grappling, grabbing, choking, locking; and throwing - sweeping, reaping, flipping, and other take-downs).
What many people refer to as different "arts" or "styles" within the Martial Art, I call systems. The reason being that I believe the "Art" is all knowledge that exists, and all options available. No one can teach it all, or learn it all in one life time, but we can study a system that contains sufficient responses to successfully address all of the general groups of attacking options.
Some systems limit their categories by not working on the ground, or by not fighting close range, or against a long range kicker. This is where people find "gaps" that they want to close. This is what some systems have evolved into - - mostly by the choice of Founders and subsequent instructors in each generation. It is not the "Art" that is lacking, but the choice of the instructors and participants in any given "system" which chooses to weed out certain techniques or entire categories of techniques.
From a perspective of Taekwondo, I view this as a word that labels where it comes, and a philosophy for strategy more than the specific techniques contained within. The term "Taekwondo" was suggested in 1955 to give a name to what already existed. The word does not make up the Art - - The "Art" is what we do in response to an attack. Training in the Art is being prepared for that response. In recent times, people have said "I like the kicks of Taekwondo," or "I love to compete in tournaments, so that is all I want to focus on." The combat aspect of Taekwondo/Hapkido/Hoshinsul/Yudo is lost to them (eg: close range hands and elbows, take-downs, joint locks, pressure points, ground-fighting). Taekwondo is not void of these things, but some schools/instructors choose to proceed without it and their definition of "Taekwondo" becomes the publics perception.
In the history of any system of the Martial Art, I don't believe that any Master said to his disciple, "wait, you can't do that move because it is not part of our system." What they most likely taught and practiced was what worked and how to be prepared if an attacker did "A," "B," or "C." You don't have to be as good of a kicker as your opponent to have successful defenses against a kick, but not knowing how to kick at all, limits your options, and might prevent you from fully understanding what kickers do.
I don't have to be the best at throwing to prevent a thrower from defeating me with one throw. I can spoil their power, and know only a few counters that allow me to survive their throwing skill, and then resume with my grappling, or punching, or kicking techniques that I am good at.
I really do not believe that we have to run out to a Judo school (or invite a Judo instructor to our Dojang) to train in throwing, however research must be done to learn it right. I have seen many Taekwondo instructors who threw with poor skill - - it doesn't have to be that way. I don't have to become a Black Belt in jujutsu (jujitsu, juijuitsu, etc) in order to perfect my ground-fighting skills within my Taekwondo training. Nor do I believe that an Aikido practitioner needs to start doing jump spin hook kicks in order to be adept at avoiding the impact of a kick.
The only "gap" in any system within the Martial Art, is not having a adequate repertoire of responses to whatever your opponent throws at you. Observe other people doing other methods and systems. Train in them if you want, but I do not believe you need to go outside of your own system to close those gaps. Just find out what those gaps are and train to prevent the penetration through that gap.
That's my personal opinion.
CM D.J. Eisenhart