Even without beating them he would've gathered experience from those fights. It looks like he now is about to start doing something with awakening. It won't be as if Luffy would be on the sidelines in that situation I would see that as a Marineford arc 2.0. And if he is facing BB he has all the time till the end of the series to get to the point where he needs to be to beat a yonkou as opposed to right now.Pitting the Yonko against each other and making them take out each other is a smart idea but it also would be waaay too easy and we both know that's not how Oda rolls. BB taking out Shanks is 100% happening though but that's the only one imo.
Also it doesn't do anything for Luffys progression if they just take out each other, like how will Oda be able to explain Luffys massive jump in power when he faces Teach if we go by your theory? He would not have even fought a single Yonko up till then but yet he's strong enough to beat the strongest one which would be Teach when that time comes with no former Yonko experience at all?
There are still some hurdles between here and Raftel. Lots of strong friends and foes that can teach him some new things.