Aggron's future

i'm in the same position, I've just kept a few lucky high-IV and -level aggron and aron (for future evolutions in case he gets a move rebalance)

Auto-select ingame tends to prioritise TDO, not DPS. That's why aggron appears everywhere. It's bulky, and has a combination of types usefull in many situations. But yes, although I like aggron a lot, let's face it: nowadays It's crap for raids/gyms.
In the future it gains a mega evolution with much more defense, more atk, and a pure steel type, becaming a monster. But until then, I see no future for aggron other than making a kirlia solo withouth breaking a sweat

As TulipsRenegade said, Aggron doesn't really serve much of a purpose right now. It's a pretty awful attacker in terms of DPS, but given that it has high defense and a bunch of resistances, auto-select just loves recommending it for just about everything. As a defender, it doesn't fare much better given that it has a double weakness to fighting, so attackers will quickly plow through it with Machamp.

My recommendation is to make two battle parties for the any current t5 raid boss (your A-team and the backup squad) so that you can just ignore auto-select entirely, apart from it giving you an idea of the moveset you're up against based on what it picks.

The other option is to toss your Aggron(s) into gyms that either you don't care about or have low turnover and don't revive them if they come back, preventing auto-select from picking them.

Don't transfer them. They are cool. Make them faint so they won't appear in the auto-select list.

In a silph league that excludes ground and fighting, Aggron could be a powerhouse. Dragon Tail is a solid pvp move, and heavy slam and stone edge have great type coverage

I quite like aggro in masters. Makes Mewtwo ez pickins