Miracles was a dominant deck at Grand Prix Columbus with 4 copies in the top 8 and 7 in top 16. Miracles also made the finals in Grand Prix Prague. Because there are a limited number of major Legacy tournaments, it is harder to make ban decisions based on results. The results from Columbus and Prague suggest Miracles may be too consistent of a control deck. The other concerns surrounding Miracles are deck diversity and the sensei's top time issue.
Banning top solves both the issue with consistency since fetchland + top allows the miracles player to look several cards in a turn and easily find answers such as terminus, and the issue with the clock. The argument against banning top is that Miracles may cease to be a viable deck choice, and counter-top combo has been removed from the format entirely.
Two legacy archetypes have suffered from the popularity of Miracles. Miracles has replaced a good deal of stoneforge mystics decks. Counter-top allows for a better combo match up, and terminus in combination with swords to plowshares enables Miracles to have more removal while dedicating fewer slots to win conditions. The archetype that has suffered is elves. Terminus is incredibly difficult for elves to beat. A terminus ban would make the creature match-ups with Miracles closer, and it would force players to decide between playing counter/top and stoneforge/batterskull.
Another concern with Miracles is that the decks that are supposed to beat it, are often discovered to not be quite so favored after further testing. Shardless BUG was often seen as a reasonable favorite against Miracles, but many experienced legacy players argue that the match up is close, and some argue Miracles is a slight favorite.
While many players see sensei's divining top as the culprit and not terminus, I would argue for a terminus ban. While chalice of the void and counterbalance/top are not always fun to play against, they do offer players tools to punish decks like storm and delver than load up on 1-drops. I think having counter-top combo is healthy for the format, and allows Miracles to be a competitive deck option. If terminus is banned, elves and other creature decks will be slightly stronger deck choices, and the power level and consistency of miracles will be brought down a notch. A one-mana, instant-speed sweeper is not fair. Had the miracle cost on terminus been 1W we would most likely not have seen the same dominance of Miracles that we saw during the last legacy GPs. Since we do not live in a world where we can modify cards, it is best to remove terminus from the format.
Some people make the argument that legacy is healthy because there are so many deck choices. This argument holds true at many local store's legacy events, but at a GP you can expect to face Miracles at least once or twice during the event, and if you plan on making the top 8, you should avoid playing a deck like elves that is going to lose to a good miracles player the vast majority of the time. A lands player will lose often to storm combo, but they are going to be favored against a lot of creature decks. Miracles is equally good against combo and aggro. Banning terminus will force Miracles players to make more deck-building choices and sacrifices in order to choose which match ups they wish to be stronger against.
If you have any questions or comments, I'm happy to expand on or debate possible solutions to the consistent performance of Miracles.