You have to look at Pokemon as not just independent units but as opportunity cost versus other options. What does Aggron do well that Stakataka doesn't do better? Aggron just has basically strictly worse stats, worse abilities (Sturdy is your best bet and is just a really poor fit for a super bulky Pokemon like Aggron whereas Beast Boost is great), and it has a pretty big movepool but really nothing of particular merit that Stakataka doesn't have (and not having Trick Room is a big negative!). Head Smash + Rock Head seems cool, but remember Stakataka has more base attack so its Stone Edge will not do that much less damage whereas it will do a lot more damage with moves that are not rock moves and running Rock Head means you basically don't have an ability except insofar as you are fixing the awful recoil on Head Smash. The best thing I see that is unique in the Aggron family is Endeavor + Sturdy, and while that's a fun gimmick, it's again just a terrible match for your awful speed tier (unlike Stakataka, you don't even really have good TR speed; base 60 is basically bad in every situation) and Endeavor is again just an awkward signature move for a bulky Pokemon.
So yeah, you could force Aggron to work on just the right team that covers for its many weaknesses and can set it up to take advantage of its high defense and/or its big offensive movepool, but in almost any team you build, Stakataka would do its job just plain better. Assuming your opponents are good players who build the best teams they can, using a Pokemon that's just a worse version of another option is just not something you can afford to do. Aggron is really bad, and trying to make it work is really working against your team's chances of success.
