A "martial art" that actually functions as effective preparation for real combat is an extremely rare thing. So any definition of "martial" for which combat effectiveness is an essential criterion must be discarded, or else we need to come up with a new name for 99% of martial arts.
With all that in mind, my definition of a "martial art" is one (a) whose techniques are descended from an origin in physical conflict, and (b) which is ideally practiced with the dangers of physical conflict in mind.
Having practiced Martial Arts for years and then eventually integrating them into what I do in the Military, and being trained in CQB and Combatives, I have found it to be a multi-faceted thing with about as many opinions and perspectives as there are people.
What prepares people for "real combat"? hard to say and my opinions changes frequently. There are techniques, tactics, and proceedures (TTPs) which will vary depending upon conditions, experiences, and technology etc. There are foundational skills that don't seem to change such as being in good shape, the ability to move efficiently, and the ability to think clearly in the fog of war.
So, does parkour training qualify as a martial art? These days I think someone well versed and in good shape in parkour would do very well in combat or in hand to hand given some basic training in the TTPs required of H2H.
So, I think the realm of what constitutes a martial art can be quite large, and of course, Aikido CAN fit well within that realm as long as it is trained with the proper focus and perspective. As I stated, I believe that two guys can be side by side and get entirely two different things out of training.
So, is there a need to be able to execute a Shionage in combat today? most likely not, however, a good understanding of the mechanics of Shionage can be very helpful in understanding things martially.