Frank Karsten is one of the most correct people I know and he recently said that unbannings are more exciting than bannings. That means we can just accept that and I don't need to write a billion page essay on why I deem that to be the case. In that light I got to wondering if the time was right to unban anything for the cube. Rather than try and build suspense for the benefit of the few who fail to register the title we will just confirm that Library of Alexandria was the card that seemed most suitable for re-entry.
Another aspect regarding the balance of the cube is that three colour or more decks are super good at present but can least afford colourless lands. As such a card that is best in decks with lower coloured mana requirements and has a very high power level has some effect towards balancing the meta in the desired direction.
Another reason for adding Library back was simply the desire for more colourless producing lands that are exciting in themselves rather than just quite good lands that are colourless sources. I looked at Stripmine briefly but whatever criticisms you might have against Library, Stripmine is a far worse card. The free wins it gets are far less of a game and the Strip Mine is never dead, it isn't even a bad land and thus a real cost to your deck as you can basically always use it in spell mode.
The final thing that allows me to get away with putting such a wildly powerful card into the cube is that Library has actually gotten quite a lot worse in recent years in a midrange cube meta. Tempo is huge, if you spend all the uses of your first land drop to draw cards and not affect the tempo of the game you will just die to a lot of the decks out there. There are also a lot more good ways to sink mana throughout a game meaning you don't have those spare mana turns where you can sneak in some cheap extra draw nearly so often.
I just did a 2v2 team sealed in which our team got Wastelands and some other various ways to counter Library (which we knew they had as we were using the whole cube). Neither of us had any answers to Library in either of our final lists despite the many options we had simply because we didn't fear it at all. I had a very aggressive Boros deck that had way too low a curve, like 14+ one drops, to play the Wasteland as anything other than a spell. There was no real synergy with it and how the deck was working and it just seemed best to do what my deck did as best it could and as consistently as it could. My team mate had a midrange Esper style deck that was more vulnerable to the Library in general but being more control in nature it wanted to be increasing its own mana count. It is rare that you will be anything other than behind if you use Wastelands to kill a Library as half of the time they will get at least one activation out of it. As such I would rather not combat the Library directly with a Wastelands but instead spend that mana to get ahead in other ways.
Finally I want to have a quick look at Sea Gate Wreckage and how that compares to Library. They are the inverse of each other in some ways and mostly the same in the rest. Sometimes doing this makes for a weak card, say Tarmogoyf and Ghoultree with the latter being a bad inverse version of the mighty Tarmogyf. Then there are other inverse pairs where both are great cards, they just find themselves used in very different kinds of deck. An example of this kind of pair would be Treasure Cruise and Ancestral Visions. I feel like Sea Gate Wreckage and Library of Alexandria fall into this latter group of pairs in which both are decent but that will be playable in the opposite sort of decks. The inverse difference between the two is Library needing a full hand and Wreckage needing an empty one. The three mana cost on Wreckage is an attempt to balance it somewhat and indeed if that cost were on Library the card would be unplayable. Luckily the nature of Wreckage is late game where you will have mana spare and so the cost, while annoying and significantly de-powering, is not crippling for the card. Ruins will see less play than Library because there are lots of options for late game lands that yield value of which many are safer or more reliable. Ruins needs you to have more colourless sources in your deck while being itself a colourless source. Even so, it remains playable in any deck expecting to empty its hand. This could be RDW, affinity, big red style decks which are cheap or explosive. It could also be very slow grindy decks that do not have many situational cards that will sit in hand. I fully expect Sea Gate Wreckage to see little play, certainly compared to Library. Despite this I have a sneaking suspicion that the Sea Gate will perform slightly better when it is used than the average performance of Library.