Some of the oldest examples of martial arts are sport. Basically wrestling.
(Image taken off a really old wall)
Sport is a method of resisted training that allowed a person to engage in violence without the drawback of being crippled or killed.(mostly).
Sport training is realistic provided you accommodate three issues sport faces.
They are fighting back.
They are about as good at fighting as you are.
Nobody learns if they are getting hurt all the time.
So to accommodate those issues sport requires rules and refs and all sorts of limitations placed on the fighter.
But are these limitations such a bad thing. Without the advantages a person must adapt what they have. For example I mma.
I use striking as an advantage to make throws easier. But I also wrestle where throwing someone is harder. Limiting my option in training forces my skills to increase.
When guys fight camp. They fight fresh person after fresh person denying themselves more advantage in training.
This way when there are more options I am at an advantage over someone who has trained buy giving themselves all the advantages in training.
Train hard fight easy is the motto of competitive martial arts.
In competition winning by incapacitating your oponant Is a very real prospect. This despite the gloves the rules the safe environment the ref and the other guy who is a trained fighter who knows he is going to fight and has prepared months in advance.
The amount of over engineering your self defence to achieve that result is quite phenomenal. But also should you wish to compare competition to self defence. It works exactly in competition as it works on the street.
A boxer can incapacitate an attacker even if for some insane reason he decided to defend himself with gloves and following every rule of his sport. It works exactly as it says on the box.
And competitive fighters don't have to. They can take advantage of hard pavement bare knuckle illegal strikes numbers,weapons,de escalation and all the advantages that a person has in a self defence situation.
So certainly competitive martial arts are viable as self defence and athletic pursuit as they were when they were invented. Which was basically as old as recorded human history.