I do know more than one person that is pretty darn good or that I would consider "shihan" level that have pretty much gotten there with only an aikido background.
I don't think you can draw a correalation necessarily based on previous study in other art verses none.
Well, that's my point. It seems that all of whom we might call the Aikido Greats today came with their own martial arts experience. The Original Poster can do the same
I agree that there may be some arts that are closer to the techniques of aikido than others, and that there may be arts that are closer spiritually to aikido than others, and that there may be some that better develop the internals / externals / or whatever other aspects in preparation for aikido than others.
Of course it depends on which style of aikido you're drawn to, and which teacher, and so forth. And of course if you study judo, someone will say that we have to undo all of that muscling and readjust your ma-ai; if you study gjj, we have to undo your tendencies to go to the ground; if you study drajj, we may have to correct your spiritual perspective; if you study karate, we have to undo your hard blocks in favor of soft parries; ...
It is what it is.
I agree that eventually it is all aikido if you are doing it with aiki as aikido is simply a structured methodology or pedagogy that encourages the development of aiki if you are training it correctly.
What is correct
? That's another story. Is structured methodology or pedagogy correct? Maybe for some, not for others. Who knows? I put on a white belt and show up here or there; guaranteed, someone there will tell me I'm all wrong!
Fortunately, aikido itself has many entrances and paths within itself to take you to the right place---wherever that is