Oda actually had a pretty clear plan for the crew going way, way back, so this assertion is actually false. The first nine, at least, were planned for a long time.
http://vignette2.wikia.nocookie.net...ly_Crew.png/revision/latest?cb=20150404102407
Here you can see everyone but Robin in some form. She was a tiny little pixie-dude with a hammer instead.
No, no that's not what I mean.
Obviously, Oda had a plan. What I am questioning is the implementation of that plan when it comes to Luffy's character. Oda may be meticulous in his plotting, but Luffy is frivolous in his decision making. Which is why there is a disconnect between the discussion going on for who would be the next crew member as part of the story (because Oda set clear patterns on what is going to happen, like each member needs to fulfill some job) and why Luffy invites people (because he just likes them)