i do think mega abomasnow should move to s. it has been said quite a few times already, but it's stupidly hard to switch into this pokemon, which means you probably wouldn't want to give aboma any switch ins. this means so much in regards to some pokemon which i would love to use, but i have to consider twice or more, simply because i might have to oppose one mon. alomomola, slowking, seismitoad, jellicent, tang, almost any electric etc. while those might have ways to hit abomasnow for big damage, it's not gonna be enough to kill abomasnow, but in return abomasnow can giga drain health back or simply kill. so you either have to let one pokemon die to get enough damage on aboma, so that it can't get off a second kill, or you have to switch out, which as said often enough most likely means something is dying (and makes the game a lot more prediction based). now it might be true for quite a few pokemon that they don't have a solid bunch of switch-ins, but mega-aboma has the advantage, that those pokemon, which allow it to get off a hit, are very vital in this metagame, often a keyfactor in the match-up, 'cause outside of letting abomasnow in, they are really valuable. alomomola and slowking, for instance, are often the backbone of a team, and important for handling several pokemon. so it's either let them die, or let something else die and maybe give abomasnow another shot at attacking.
even against offensive teams, it has quite some opportunities to attack most of the time. so they also have to pick a fodder usually, or predict perfectly, which kind of doesn't count. so it's in most cases at least trading a kill, in case of the opponent having an electric, or flygon which is forced to defog, it's probable that aboma does more than just that.
my point is, when teambuilding you will very often find yourself in a situation where you are forced to decide if you really want to use a certain pokemon, which even though it's great otherwise, allows abomasnow to attack, or you'd use something else, that most likely will make your team less effective. abomasnow is such a huge factor while teambuilding, it makes use of quite a bunch of top-pokemon, that it perfectly fits in s-rank. i don't think any other pokemon has such a huge influence on the metagame (especially teambuilding) as abomasnow has.