Fr. Enright finishes up his survey of the history of the diaconate with a very quick look at the medieval period (roughly, 700 to 1500), the Council of Trent, and Vatican II.
In keeping with what we've already said about continuity in the most basic idea of what the diaconate is sacramentally, note especially the first quote from St. Isidore of Seville and the last quote from St. Thomas (p. 20 and 22 in my edition). Even though the diaconate was more and more treated as "transitional" throughout this period, we do know that it (and the minor orders) were flourishing. St. Isidore gives it a very significant degree of respect, and St. Thomas's theology (although shaped by his very heavy concentration on the priesthood and confection of the Eucharist, typical of the time) articulates a clear "sacred power" to proclaim the Gospel, both liturgically and, I think, ministerially, which the handing over of the Book of the Gospels in the ordination rite signifies. Nothing here seems to indicate an order in decline or being marginalized in the Church, despite the relative lack of "permanent" deacons.
The Council of Trent considered, but did not in the end enact, a revival of the permanency of the diaconate. It stated unambiguously that deacons belong to major orders, but it did not give any clear definition of the deacon's sacramental identity.
In the 400 years between Trent and Vatican II, something significant changed, though, and this article (and the next, really), don't manage to identify that change. I don't know the reasons why, but two things happened: (1) the minor orders essentially disappeared from parish life; and (2) the norms of celibacy became much stricter. Those two things are probably related, in fact, but I haven't seen any research into how. What this means, though, is that the context of the "renewal" of the diaconate in the 20th century was significantly different from the context in the 16th or 17th century, in the wake of Trent.
One way to think about the renewed diaconate is that all the functions of the minor orders, which (at least in one theory) were unpacked out of the original diaconate, have now been repacked back into it. In a sense, it's useful to think this way, because we now look at deacons much the same way the medieval Church looked at the minor orders: ordained men, pillars of the Church as it were, mostly married, mostly not employed by the Church, taking on clear, necessary, but subordinate roles in liturgy, parish life (catechist, sacristan, etc etc), and witness in the world. (Theologically, of course, there's a big difference between major and minor orders, which we're skipping over here.)
The cost of thinking in these terms is usually the loss of clarity about the diaconate's "necessity" (as Fr. Enright puts it in his conclusion, last paragraph, on my p. 26), in three senses. First, (also because of our cultural attitudes,) we lose the sense that something is missing when Mass is celebrated without a deacon ("low" Mass, though we don't call it that anymore). Second, we lose the importance of the transitional diaconate - which, as we noted before, doesn't actually seem to be a "decline" of the diaconate in the patristic or medieval Church - even to the extent of suggesting doing away with it. (One of my biggest beefs with Deacon Ditewig is his foolish promoting of this very bad idea.) Third, we lose clarity about the deacon's "sacred power" of proclaiming the Gospel in liturgy and in ministry, and therefore about the deacon's most basic identity and share of the apostolic mission.
If we can hold on to the attractiveness of this vocation (in terms of the loose parallel to the way the minor orders used to flourish?) and at the same time hold on to a robust theological precision about identity, sacred power, and double usage (permanent and transitional), then I think we will be close to embracing the "necessity... to the Church and her mission" of the diaconate that Fr. Enright lauds in his conclusion. Which is devoutly to be hoped, prayed, and worked for.