I certainly agree with the idea that there is NO such thing as "the ultimate" martial art.
The reason is that, like any other human endeavor, a particular martial art is an accumulation of responses to a series of historical, cultural and social circumstances.
Therefore, a given martial art is (was) suitable for a particular set of situations. For the same reason, some arts or systems designed in response to a particular set of situations may not be appropriate in context of DIFFERENT social and cultural settings.
This is why, I believe, many "traditional" martial arts do not make very effective self-defense systems in 21st Century United States. We do not live in 17th Century Guangdong, 18th Century Okinawa or 19th Century Tokyo. We do not have to fear the two swords-armed samurais and we do not have to fear an attack while sitting on a tatami.
Looking at it from another view, one cannot escape the conclusion then, that some arts are MORE appropriate for the social and cultural setting of where and when we live (21st Century America). Some arts such as BJJ and Muay Thai are, IMHO, much more effective in a real mano-a-mano fight than Shotokan or Tae Kwon Do. Mind you, neither is a "complete" or "ultimate" art useful for all situations.
There is also a misconception about how a martial art is to be used. Too many folks have the mistaken Hollywood notion of "get insulted, break out into the fists of fury, kick ass and preserve honor" hype.
Folks, any self-defense system of UNARMED nature is really the self-defense method of the last resort. If you can stand on your feet and have an avenue of escape, by all means, run away, flee, escape or call for help. Let the police do the "ass-kicking."
This is why I personally feel that grappling arts IN GENERAL make better "last-ditch" self-defense systems than striking arts, because when you are ambushed, tackled or forced onto the ground is the time when you will most need an UNARMED self-defense system.
Some criticize grappling arts as being unsuitable for self-defense because 1) they are unable to deal with multiple attackers and 2) they cannot cope very well against armed opponents. To those criticisms, I respond thusly: if some of the more "traditional" striking arts are not effective in a mano-a-mano fight, how effective would they be in multiple opponent scenarios? Clearly, to involve onself in a situation where many are against one is ultimately very foolish. Grappling will not get you out of such a situation, but neither will Tae Kwon Do, Wing Chun, boxing or any number of other systems.
As for dealing with an armed opponent, it is true that some arts may be marginally (emphasis on marginally) more effective than others, but unless one puts years and even decades into the training, it is again ultimately futile to engage an armed opponent while one is unarmed himself (hence the familiar saying, "it is a fool who shows up for a gunfight with a knife"). Unless one is willing to put years and decades into such a training, IMO, the time is better utilized by training in some effective unarmed vs. unarmed techniques.
For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the supreme excellence. Sun Tzu