In what world is M-Gross an opportunity cost by all of these you mentioned WebBowser >_>? Also classify a set up sweeper exactly because M-Gross has access to Hone Claws along with Agility / Rock Polish so it's silly to actually argue that he sucks as a set up sweeper when in reality it really doesn't unless you're running like 3 of the shaky checks to it all at once to combat it, making you weaker to the archetypes M-Gross will fall under. This isn't a case for anything specifically but I must be misinterpreting something if you're seriously considering that there is some sort of huge competition for a mega slot with M-Gross v everything else cause I must be seriously missing the logic you're presenting here.
Hone Claws is honestly really lackluster. Hammer arm, one of his better coverage moves, lowers it's speed upon use, which really sucks if you're going for a sweep. Bullet Punch, even at +1, is not going to allow it to bypass any of it's common revenge killers (which is now much more pertinent since mmeta bothered to take a turn to setup), and a lot of his defensive checks still check him at +1 (he's probably not running grass knot if he's running hone claws, so slowbro still works, rotom-w can still take a hit and burn back, whirlwind skarm still doesn't give a crap, etc). Basically, hone claws doesn't allow him to bypass most of his common checks while at the same time taking one of his precious free turns to setup. Compare to SD talonflame, who at +2 can bypass a lot of things that would normally check him like Scarf Lando-T, Skarm and mega Gyarados.
Rock Polish, while better, isn't amazing, because most of his checks are on the defensive side anyway. He already has the bulk to take on most scarfers, but rock polish doesn't help out at all against common checks like ferro, slowbro, and rotom-w. It does allow him to beat lando-t(assuming ice punch), but the entire point of setting up is that you hand your opponent a free turn to bypass some checks, and speed boosters beating scarfers is nothing new(remember back when agility duck-z was dominating the meta? I don't either). The most obvious comparison to make is to Mega-Diancie, who does not have the bulk to take on most scarfers and therefore really appreciates the speed boost.
And then we have these mons that can boost both attack AND speed at the same time using this nifty move called "dragon dance", some of them are pretty bulky too!(hello Mega Gyarados)
Alright, sarcasm aside, Mega Metagross is primarily a bruiser, he comes in, hits stuff, and leaves when he feels like it. In this he excels and if he is broken, it's because of his capabilities as one. He also has a few other nice utility niches like pursuit trapping, stealth rock, and fast priority (now that I think of it, why has nobody on this thread mentioned his potential as an offensive SR setter? That actually sounds pretty neat). His sweeping capabilities are lackluster because he doesn't have enough power to be an effective speed booster and hone claws is a crummy setup move in this day and age where if you aren't boosting at least 2 relevant stats at a time, you're wasting your time and free turns(which are at a premium in this game).
Anyway, as for competition, he faces competition from any mega that isn't a bruiser, so walls like mega sable and mega bro or sweepers like megazard-x or mega pinsir can easily find themselves a niche. Even as a bruiser, his typing leaves him vulnerable to common attacking types like ground, fire, or dark, meaning that depending on the teamcomp, you may prefer a different mega like mega venusaur (trades offense and speed for much, much better typing and a sweet defensive ability) or bulky zard-x (amazing typing AND amazing power in exchange for speed and defense). These megas are still plenty viable and I even run into them at times (heck, my team doesn't even run a mega anymore, but when it did, it was bulky SD zard-x).
So yes, the opportunity cost for using him is in fact real, much more so then previous mega suspects.