One of the most common arguments for Aegislash' ban back in XY was that it was very centralising. I think it is good to consider
this post and
this post when talking about a pokemon which is so overcentralising as Aegislash is. The kind of centralisation Aegislash brings is not from a offensive point of view, such as pokemon like Kyurem-White will be if they would ever be retested, but from a defensive point of view. Aegislash checks so many pokemon that are viable at the moment (the complete list of pokemon that will drop 3 subranks in viability is already posted a few times) which makes it a perfect glue on almost every team, mainly because it can run different sets to work on different archetypes.
There are two important questions regarding Aegislash' centralisaton which are: do we want this centralisation and is this centralisation healthy?
The first questions can be answered by the posts I mentioned earlier in my post whether we want a diverse, match-up based one or if we want a centralised, battle-centric one. I prefer the former, because it rewards teambuilding and while there are some matches that are decided by matchup (which is often exaggerated btw) the matches that are not decided this way are still slightly in the advantage of the better teambuilder, while still giving the one with the worse matchup a very good chance of winning the match. In most cases, the better your team is, the lesser amount of matches are decided by matchup. Teambuilding this way is more difficult, but the options of viable pokemon is a lot bigger because without Aegislash you can pick a Jirachi, Starmie or Celebi if you want. With a centralised metagame this will be more difficult because you can't easily put a Jirachi on your team because it gives the Aegislash very much momentum (not saying Jirachi is unviable in an Aegi metagame, but it will cost a lot of momentum if you use it)
The second question is if the centralisation Aegislash gives is healthy or unhealthy. I am a huge supporter of a Landorus suspect and with a reintroduction of Aegislash this problem will only get worse.
Albacore described this issue earlier in the topic in
this post. This is not the only negative effect of the centralisation of Aegislash. The most threatening pokemon in the current metagame (Keldeo, Landorus, Sableye) will profit from the return of Aegislash, while some pokemon that are currently very healthy for the metagame, such as Slowbro and Starmie, will be a lot worse with Aegislash around. While there are definitely some top tier threats that are defeated by Aegislash (Metagross without EQ, Mega Altaria, Clefable) these pokemon are already in a stage where the metagame adapted to them with pokemon such as Slowbro, Hippowdon and Scizor who are all gaining enormous popularity.
With Aegislash in the metagame, the metagame would not only be centralised, but will also be filled with many (potentially) broken pokemon which puts the same pressure on teambuilding as they do now. The OU Metagame would be a lot better with Landorus (and maybe some other big threats) gone, rather than reintroducing a defensively centralising pokemon. It can be argued that having a centralising force in the metagame will be great to lower the matchup problem but Aegislash doesn't fix this problem because the best pokemon (plural) in the current metagame still wreck the whole tier while the only thing Aegislash brings is more centralisation amongst these threats. As a supporter of a diverse metagame and a supporter of a metagame where broken mons doesn't check broken mons but broken mons are just banned I want Aegislash to
stay in ubers.